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.
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With great difficulty.
Your typical BMX use 20" wheels.
If you take a bike built for bigger wheels and put 20" wheels on it the bottom bracket(the mounting point for the pedal arms) will hang low, creating risk for pedal strike if you pedal through a turn.
Using smaller-than-intended wheels will also make it difficult for you to get any rim brakes to line up.
Even after that, you're still stuck with a bike that might not be rugged enough to withstand the beatings, the falls and crashes associated with BMX riding.
well you want to find a bmx bike for around 300+ those are usually better and longer lasting.make sure you like the colors, weight, and most importantly comfort.
Buy all all of the parts for it and built it yourself or go to a shop and they can get parts and built it for you but it wont be cheap.