get the parts and put it together. buy the co2 tank.... find a cheap regulator buy a needle valve and hook it on..... Solenoid too if you want..... though a lot of people just break down and buy one put together. they have some one ebay as cheap as 60-70 i think with better ones 90-100. yes its more money then diy co2 to start with but its worth it. one of the few things you can buy set and almost forget about.
They mean a measuring cup. Where 1 cup ~= 1/4 liter. It's actually closer to 0.2366 liters, but 1/4 a liter is close enough. It doesn't have to be exact.