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.
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.
please see http://www.YouTube.com/watch?v=sqOSRGNO84Y for full details on how to build a complete bmx. get all the parts for youur bmx then just put them together :P
One can buy a custom BMX bike in Las Vegas from a few stores. One can get a BMX custom made from 'Just BMX', 'Bolder BMX', 'Bike World' and 'Vegas Bike Store'.
Sunday
it matters what you are going to use it for
Go to bike shop or wolyworld and buy a link remover.
you can find a BMX bike in halfords
The bmx thruster rampage bike is a exellent bike
go to a bike shop and start to build it there start with the frame its the most expensive
A hyper is better
no but you can get a dirt bike or a cruiser bike or a speed bike but i havnt seen a bmx bike yet.
they are more stunt bikes but if you want to do more jumping on and of you should buy a trial bike
The type of bike that is mostly used form BMX riding are the purpose-built BMX bikes.
Upgrading will usually cost more than it will bring, but it all depends on what your current bike is, and what you want to do with it.