Answer:
It depends how many pieces it is in! Also note there are many specialist tools required.
If it is completely disassembled, I would do it in the following order:
1 - Seat post and saddle so you can clamp it into a work stand without fouling cables
2 - Main bearings (bottom bracket, headset)
3 - Forks, stem and handlebars
4 - Cranks and chainset
5 - Front and rear derailleurs
6 - Brake calipers/cantilievers
7 - Brake/gear levers and grips
8 - Cables (you should be able to adjust them correctly without the chain on)
9 - Chain
10 - Pedals (so you can spin the cranks easily!)
11 - Wheels/tubes/tyres
11 - Adjust!
Of course there is far more to it than that summary, but that would require an entire book.