If it is just the wind up part, there is a spring, a peg, a catch sort of thing that looks like a big Z or N. The spring hooks into the 2nd hole in front of the big gear that winds when you crank it. The spring must seat around the peg, which goes into the big hole in the middle. Then the Z piece goes on the peg so that the piece with the angled end, not the sloping one, is closest to the spring, and will engauge the gear, stopping it from moving. The trick is to get the Z piece on the spring while keeping the spring centered around the peg. Once you have that on, hold it all down, and click the ratchet starter once click, that will put enough tension on it to keep it from falling out when you try to reassemble it to the bike.
I hope this helps.