Start by learning how to set up your own guitar in every respect. Then buy a cheap rubbish guitar to experiment on. Next buy a guitar and completely refurbish it. Learn how to fret and fret level, learn all about neck angles, intonation etc. You Tube is a good source but I also found some very good instructional books. A Partscaster is a good intro to a full build with a lot of the specialist work (like frets) done for you. You will gradually acquire the skills to build your own from scratch.
This was my path - including restoring a couple of 1950's Hofner guitars. The Hofners taught me about re setting a neck and neck angles.
I have now been building guitars and other stringed instruments for about 7 years, learning all the time - still learning.
As a hobbyist there is no money to be made from re-selling the guitars you build however good they are as people put a lot of value on a "makers name". I manage to sell for the cost of parts plus a little - nowhere near the true value. I have come across a couple of luthiers who manage to ask and get good prices for their guitars but I know others who have given up due to the poor return for all the many hours it takes to build a guitar. I think the ones that do well are the ones who can 'sell up' their skills with marketing smoke and mirrors.