Reading. Hours of reading & searching. Then trying.
Learning something worthwhile will always take some effort.
There are a lot of builds on this forum ranging from simple to very complex.
Lists of modding gear, supplies, how to’s etc.
Some of the most documented how to threads are previous contest entries, have a look.
There is an archive of links to all the old threads kept in one place, partly why I made it, for this reason.
PinkPandas Light building tips:
Have an idea and search the modding forum, someone has done it or something similar most likely.
If you’re stuck, ask. These dudes are helpful.
I remember the days of thinking to myself that I would never bother to do a driver or emitter swap, look where that went… 
It doesn’t have to be perfect, perfection doesn’t exist in human terms.
Everything I’ve made has flaws of some sort.
Call it makers character 
Most things I know about flashlights are learnt from people here, new things are picked up every day and still lots to learn from others.
We all start somewhere, the hardest part is deciding to just do it.
I won’t lie, there will be some heartbreak & frustrations along the way but your successes are an achievement only you can do and make you happy in ways like no other.
Remember passing your driving test, exams, cracking that contract at work… that sort of thing.
Only you can decide to do it.