| Get copies of Dan Erlewine's books "Guitar Player Repair Guide" and "How To Make Your Electric Guitar Play Great", get on Stewart-MacDonald's mailing list to get hard copies of the catalogs (which have a lot of great tips), read through the stickies at the top of this (Hardware, Setup & Repair) forum, John Caruthers' "Teach Yourself Guitar Repair and Maintenance", and "Complete Guitar Repair Guide" by Hideo Kamimoto.
Erelewine is pretty much the current ruling sage and guru of repair, set-up, etc. I love his books because he really delves into the trade-offs inherent in choice one needs to make when doing set ups. The books are full of excellent graphics (photos, line drawings, diagrams, whatever gets the information across most directly), and are easy to read and follow, plus he devotes a lot of information to basses in particular. Carruthers ran or still runs Valley Arts Guitars, and he helped put together Leland Sklar's studio workhorse bass (the P-shaped body with the two EMG Precision PUPs, the P neck shaved to Jazz bass specs, and all the autographs) as well as building necks that Will Lee used for a long time. I learned a ton of useful stuff from his columns in Guitar Player Magazine back in the '70s and '80s. Kamimoto was a prominent bay-area repairman who did a lot of work for (among others) Stu Cook, Tom Fogerty, and John Fogerty of Creedence Clearwater Revival. When I started doing this stuff, his was the only book out there and I found it extremely helpful in both leading the way and being encouraging to me that I could actually do this stuff without screwing things up.
Get the books and read through them. I always recommend that anyone who wants to start this for themselves get Erelewine's "Guitar Player Repair Guide", and read it once through with all instruments locked up in their cases. Then read it again, and make a list of basic tools (the instruments are still locked up). Then read it with your bass in your hands, looking at how things work and are put together (but the tools are still in another room!). Only then do you get the tools and the bass and the books together. So some simple things and see how they interact- like how if you have a perfectly straight neck it affects the action, and your bridge settings. Learn to write down what you're doing so you can get back to where you started, and start with changing one thing at a time. So, if you turn the truss rod 1/4 turn clockwise, don't mess with the bridge until you've spent some time seeing what that did all over the neck of the bass.
Since you specifically said "pro" I'm also recommending the other books- the "How To Make Your Electric Guitar Play Great" does have some carry-over from the bigger Erlewine book, but it's got some stuff that's not the same as well. I find both useful. Then after you've gotten to know YOUR bass really well, invest in some inexpensive but fairly well put together instruments. I don't know brand names 'cause I don't hang out at music stores, but get some stuff to practice on. You'd rather mess up the fingerboard while refretting a Chinese Peavey than doing it on a Fullerton-era Vintage Series bass.
The reality is that if you're wanting to do this professionally, then you're going to have to know guitars too (those bass-shaped objects with six or twelve skinny strings). So you need to know how a Tune-o-Matic, a Strat-style, and various Floyd Rose bridges work too (I don't do this for a living now, so I can generally blithely ignore the monumental stupidity of both the Floyd and the Bigsby), as well as at least some basic understanding of the differences involved with working on flat-top acoustics.
It's fun, and if you have the patience to get good at it, it can be profitable (not lucrative generally, but profitable).
Have fun with the journey!
John
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JTE
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Lakland Owners' Club # 248
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