I build wheels for a living. Before i built wheels for a living i built wheels for myself and a few friends. I did it mostly by instinct. I started off by copying wheels that had been built by a professional.
I would echo
@August Bicycles's comments. If you work for, or as, a custom wheelbuilder the actual building of wheels might be less than 50% of your time - after you've dealt with phone and email enquiries, invoicing, stock ordering etc.
I agree with many of
@synchronicity's points except his suggestion that Sapim spokes are inferior to DT Swiss. Both brands are excellent.
Assuming you can pick up the basics there are a few things i think are really important that i have learned since working as a wheelbuilder:
1 DT Swiss spoke wrench all the way - it grips nipples very securely.
2 Spoke prep and lubrication. Dipping the spoke threads in linseed oil helps keep them moving as you raise tensions and helps prevent them seizing long term. Lubricating nipples with a light oil through the spoke holes as you build really helps get up to tension.
3 Stress your spokes. We have a hydraulic press in the workshop, but you can achieve similar results by really squeezing groups of spokes as you build. Once wheels are at tension we leave them overnight and then press them again the next day. Then we tension them up again.
4 A tension meter is very useful. Even a cheap one should tell you if your tensions are relatively even. Correct, even tension is key to building a strong wheel.
My advice is to go for it. I am still riding wheels i built 15 years ago in a state of blissful ignorance.
Rich