The process is simpler than you think. Go get an alkaline AA. Solder appropriate length wires to it (5-10in, enough to get it out of your way). Also solder some wires on the LED’s MCPCB (long enough to get it out of your way, preferably under a sheet of paper or under a paper cup or whatever). Solder the LED wires to the circuit, then solder the AA to the circuit. Now you can work on the PCB with your DMM.
You’ve got 3 options for mode switching:
- Do what I described above and be sure you’ve got it in the right mode before final soldering
- Cutting a wire to the AA, stripping the ends, and tap them until it’s on medium/whatever - then twist them.
- I’d just wire in a switch.
Do not power up the driver without an LED attached.