Attach the two side supports to the baseboard using the L-braces. Place them as close to one end of the baseboard as the L-braces will allow. Make sure the L-braces are in-between the side braces and the edge of the baseboard (like in the pic). They need to be on the backside of the catapult for maximum support. Make sure both side braces are sturdy and firmly attached.
For me, the L-braces alone were enough to secure the side braces down, but if you have smaller L-braces and they don't quite do the job you can improvise with screwing the side supports into the baseboard (just be careful not to split any of the wood)