Here are four pictures that will help you to get the four related parts assembled properly.
The first picture shows the four related parts. In counter-clockwise order, they are the hammer, the hammer retreat that backs the firing pin away from the cylinder, the trigger rebound that returns the trigger after firing, and the trigger.
The next picture show the hammer retreat, and the trigger rebound inserted into the back of the trigger.
The next picture shows the hammer, the hammer retreat with the trigger rebound nestled inside of it, and the trigger. Note that the hammer retreat and the trigger rebound, assembled together, will side down onto the same stud, more/less simultaneously. It instructive to practice sliding the two parts, assembled together, onto the stud, just to get the feel of it. Its best to put a drop of oil on the stud before sliding the assembly onto the stud. The two stud holes need to be concentric; otherwise it will not slide down the stud.
The full assembly process, then, has the nestled retreat and rebound parts, with the front end of the rebound bar inserted into the back end of the trigger, and all three parts slid onto the two studs simultaneously. Its also necessary to use one finger to hold the hand rotated counter-clockwise.
The last pictrure shows all four parts in their approximate positions, when they are in the lockwork area.
If you have further questsions, feel free to ask !
Good luck, Mike