http://www.bushcraftuk.com/forum/showthread.php?t=53806
An old thread but the second picture showing it being drawn and aimed is really good. I would suggest going for the same type catapult and bands as our man Siberianfury, as that simplistic design is easy to use and not bulky like those dated wrist braced things (think black widow..yuk!). The bands I'd recommend are theraband gold, ebay has loads of the stuff and it's cheap and easy to cut to shape and tie on.
You can pick up a nice wood or high density plastic frame from £10 and up, so no need to feel the urge to spend all of your pay cheque!
This guy is really something else, English guy too and sells all the stuff you might need if you wanted the same as him.
https://www.youtube.com/user/OLDMANSBALLBAG/featured
Great tips and advice to be had there also, helped me out when I was unsure on a few bits.
Other than those basics, there is all the code words too .. Main ones to know are OTF and TTF. OTF means Over The Forks, and is when the elastic and pouch is configured and tied upright in a way that makes it fling over the top of the forks. TTF means Through The Forks, meaning the elastic is tied on the sides of the frame and thus the elastics and pouch are flung through the actual forks themselves. I find OTF better for smaller frames for obvious reasons like screwing up and hitting the forks, or your hand!
Tonnes of different types of elastic to choose from, generally speaking bands like theraband shoot fast, hard and well, but don't last as long as other types. Also they deteriorate in sunlight!
Tubes are long life, harder to pull but still effective, especially since newer better rubbers are being used for them these days compared to the crap way back. Still not as good as theraband imo, and way more expensive.
And square elastic, like the old milbro style. Again, like tubes they have improved a whole lotand are hard wearing, harder to pull and harder to attach than bands on simple frames.. Different colours and thicknesses etc differentiate the strength and all thickness, but I have little to no experience with them.
If I were you, I'd buy a pre banded frame you like the look of (from a decent reputable seller, not from china) and a bag of 7mm steel shot to hit cans with, and geek out from there!