And to expand on Tom's post, the reason you heat the lower leg first and for longer is because the bottom chip is already soldered to the board, and it takes more heat there to get the solder to flow. If you try to heat both the upper & lower legs at the same time you'll cook the upper one before the solder wicks onto the pins and across the gap. Both pins have to be hot enough for the solder to flow.
Even with tutorials it’s difficult and ugly at first but you will get better if you keep trying. Keep the solder wick handy so that you can start over when you’ve messed up. The chips are pretty tough.
Before about a year ago I'd never used a separate liquid flux, I figured 'well it's flux core solder, right? So why won't it stick??!'
Now, if I didn't have a bottle of flux on hand, I wouldn't even consider trying to solder anything. Life is too short for that kind of frustration.
Sorry to jump in, but posts in this thread have encouraged me to try again… So thanks!
Here’s my solution; use some POTW [Plain Old Telephone Wire] the 4 conductor type w/ 20-22 ga solid cores. Strip some short lengths. Flatten 1/4” [5-6mm] of one end. Tin the flattened wire end. Stack chips and tin the leads. Use the wire as a handle and press the flattened end to the chip leads. Touch hot iron to flattened wire. Presto! Now you can snip off the excess wire and do the next leads.
Really sucked at it forever. But when I switched to the .031 flux core solder things started changing. Then I got a Hakko 888 station and it’s now easy. I don’t use flux. I don’t use a small tip. I use a big round angle point tip that came with the iron. I’ve tried others, but that’s the one I like best. I don’t pre-tin the existing chip. But I’ve found what works well for me and when all is ON, it’s a matter of seconds to add a chip, in minutes I’ve got 16 chips on the Qlite, or even 24. 
I hold the chip in place with the tip of my hemostats or tweezers. I don’t clamp it down, just hold it manually with light pressure. And I don’t use helping hands or magnifiers. Usually solder the outer leg, either side, first, then I still use the tool to hold down on the chip just to make sure it doesn’t shift. After soldering the 3 legs, I solder the outer ground. The legs are done by picking up a very small amount of solder off the .031 solder roll, pretty much a touch of the iron. Then I press the point onto the base leg for a second or two like Tom said, then rotate the iron into contact with the stacked chip leg and slide it up. Presto! Done. I usually watch for the solder to liquify under the base chips leg before bringing the iron into contact with the stacked chips leg.
That said, there are times that many chips go easily and you feel really good about it. Then all of a sudden one leg just simply won’t cooperate and it’s bamboozling! lol
I’ve done 40 chips in a row and then had one that took as much time as all the others combined. Beats me!
But I don’t do that any more. If I want a lesser driver output I might stack 4 chips. Anything other than that I use the FET driver and control output with PWM. Like, instead of 255 for 100% I might use 190. I usually figure the percentage to get an estimate on the amperage and go from there. Most of mine, however, are full on Turbo modes at 6A or more on the top cells. I’ve found that if I want closer to 4A I can just a lesser cell. 
Post # 29 in this thread (by TomE) really helped me when I was learning.
(edited to make direct link to post)
I do essentially what TomE says, but you might find it easier to break off the middle legs off to stop some bridging.
I don't presume to know your knowledge and skill level with regard to soldering, but I was a complete noob (still am) and learned a lot from this video series:
Simply from watching these videos, my soldering improved a lot. Hope that helps.
Oh yeah…first few attempts at this failed miserably…found that a perfectly clean tip (I run mine in a brass wool cleaner bucket) with a tiny blob of solder and a quick wipe on the leads with ample flux bridges quickly, if the tip is dirty solder has a mind of it’s own…wrecked several drivers learning how to do it right
My method
A. bend legs 1 and 3 down gently
B. tiny dab of super glue gel (10-15 second bond time) on top of host chip, place 2nd chip and hold till bonded
C. apply a good amount of solder flux to pins being soldered
D. clean the bejesus out of the tip till it’s shiny shiny, apply a small blob of fresh solder to tip
E. quickly touch leg needing soldering at the base/pad then drag tip up and away from the driver, bond is usually instant and works well
Doesn’t hurt to have some of those magnifying glass headband either even though the focal point is only like 3-4”
I bought one of these from FT awhile ago:
https://www.fasttech.com/p/1398900
Head-Wearing Elastic Band Magnifier with LED Illumination
The light(s) are a joke (dim as heck), but it has a bunch of lenses you can click in, and, with different powers, the focal distance is different, i.e. at 1.5x, it’s like 8” or something.
I had eye surgery earlier this year, and my close vision sucks now, and I don’t think I could solder anymore without these babies!
I like the superglue idea!
I’ve only done a few, but I bent the legs down, which helps keep the chip in place a little, but makes the gap small enough for the surface tension to keep the solder bridged without having to glob on the solder. My first one was so bad, I don’t know how much solder I went through. It became my practice piece because I was certain I’d probably burned the chips up on the inside from how much I reworked it! But yeah, bending the legs helps, and superglue sounds like a great idea, because that’s the hardest part is keeping the tiny part perfectly positioned so that you can tack down that first leg.
All good, but I simply put the driver in a desktop vise (HarborFreight), and hold the chip down with a pair of bent tweezers, while soldering. As long as you bend the 7135 legs down, the tweezers will hold the 7135 in position well. I'd rather not use a super glue because with heat, it can produce some nasty gas.
I got a NiteCore HC50 headlamp I use now for soldering, etc., plus magnified glass's, drug store types - couple of strengths. Think I got them from DX, Manafont -- one of those at the time... The HC50 is fine on the middle setting (3 of 5) and doesn't get hot for the full time of a 18650 discharge. On settings 4 or 5, it will get hot...
Yeah, superglue sounds like a great idea right up until you start soldering and fall over in a coughing fit because you inhaled cyanoacrylate vapor.
Just use an alligator clip.
Hmm, yeah, cyanoacrylate vapor is probably not as soothing as rosin flux. Nothing more relaxing than rosin flux wafting past your nose as you harmoniously join a tinned wire with its mating thru-hole. Once you get one leg tacked down, you’re in the home stretch, since if you do one leg at a time, the soldered leg should keep everything in place (unless you linger too long on one joint. You don’t have to let it linger. Or you’ll fry chips like me)
On the topic of glues and SMD soldering, I’ve seen some boards with SMDs where the components have not only been soldered, but also glued down with either red or clearish-yellow glue, very hard stuff. This stuff just some kind of non-outgassing, high temp epoxy?
One last thing I tried but gave up on was using thick solid core wire as a spacer for the tab. Once I get the two chips’ tabs soldered together, I figured it’d be a cakewalk soldering the two legs, but this ended up not working as well as I had hoped. The wire ended up being more like a very smooth wheel, and whenever the solder liquified, the chip instantly moved somehwere where I didn’t want it. However, I did have good luck using a thinner wire as a temporary post to tack the other chip’s leg to, solder the opposite leg, remove support post and solder that leg, and finally, do the third connection.
Would have been easier had I had some aligator clips or something like that to help me out…. Then, I wouldn’t have bothered with all the other methods of retaining the chip’s positioning long enough for me to solder. Alligator clip would have been easiest, and added benefit of extra thermal mass might buy a few more seconds of rework time
Joining the tabs is easy. Turn the PCB vertical, with the tabs at the top, and drop a blob of solder into the gap. Hold the iron there until the solder sucks onto the tabs, remove the iron, and you're done. If you use both gravity and the solder's surface tension so that they're working together instead of trying to pull the solder where you don't want it, it goes a lot easier.
Nice on those interesting glasses…they look MUCH lighter than the ones I got from Amazon, and yes the cheap o’ light on mine are woefully weak…this is why I bought a cheapo elastic headband, and either use a Sipik SK68 or one of my Convoy C2’s in it for light (the T6-4C makes everything much more distinct and the colors more brillian)
That sounds like a good tip, I’ll be sure to give that a try next time I need to stack 7135’s.
Yes I had to learn this too, the hard way.
CA gas in the eyes and fingerprints are visible forever where the fog goes…
And if someone thinks Fujik would be suitable…no that doesn’t work too.
Alligator clip is easy, everyone has them and with two free hands it is much easier to solder.