OK, let's talk about the trigger guard and sear. You will need what is called a "slave pin" (a term I am told is not politically correct and should be called a "helper pin"). This is of a length the width of the trigger guard and holds the sear and sear spring in place while installing the trigger guard. Once the trigger guard is installed, the "helper pin" is driven out by the rear trigger guard pin.
Now to the hand/trigger/lifter assembly. First, the hammer lifter and transfer bar are the same part. The spring must push the hand forward and the lifter back at the same time. The spring goes around the pin in the trigger (there is a gap) and the short end has a bend that goes into a small hole in the hand, with the other end bearing on the lifer. Assemble the whole thing out of the gun and get it working before you try to assemble it into the gun.
Then install the hammer without its spring. Pinch the hand and lifter together and insert the trigger/hand/lifter assembly into the gun with the hand and lifter up between the hammer and the frame.
Use a work pin (a tool a bit smaller in diameter than the trigger pin, but longer, so you can easily remove it) to hold the trigger in place so you can work the action. Make sure everything works, then drive the work pin out with the trigger pin.
The last thing to go in is the mainspring (hammer spring).