I start with a basic M check - what is wrong with the bike. List the things that need doing. That means a close inspection of the frame for damage/cracks - if the frame's a write off the bike is a parts donor only and stripped.
I start by removing all the bad bits and decide if they would be serviceable after a rebuild or if they need replacement.
Once everything is off the bike that doesn't work, I make a judgement call about whether its worth fixing at all. Avoid sinking a lot of time into a bike if its marginal. There are a lot of bikes out there, and while sentimentality and history helps move the boundary you still need to be selective.
Work on each subsystem in turn. IE I'd make the BB perfect and then move on to cranks. Then rear wheel, cassette, derailleurs, chain.
Always clean things before working on them. A clean bike is nicer to work with. So do the dirty parts first - that's transmission, wheel hubs, wheel truing etc.
Leave finishing details till later. Saddle, bartape, reflectors, inner cables for brakes/shifters, and brakes are toward the end of the sequence.
Finally, test the bike. You should ride it for a few minutes, slow at first then getting up to speed and testing the brakes and shifting.
After 4-6 weeks you will want to tweak the brake and shifter cables too - they bed-in with usage.