Replace the piston, mainspring and guide into the cylinder and then the cylinder sleeve. Compress the spring and locate the sleeve with the cylinder pin/trigger pin or a slave pin/punch.
The aluminium trigger is pretty much the same idea as the early triggers, with the 3 floating bit of sear. It is a bar steward to get the trigger pin through the all the bits. On older German Dianas, they had 2 pins, 1 to hold the sleeve and one for the trigger, which made it easier to fit. This is one reason I do not like working on the Milbro versions. If you are lucky, you may get the pin through first time or you may be unlucky and spend 30 minutes swearing at it. I made up a small aluminium slave pin to put through the trigger and sears first to give me a fighting chance of getting the pin through and it went in with a bit off jiggling.
Replace the barrel and locate the cocking arm in the cocking slot of the cylinder and replace the cylinder end cap and drop the action into the stock (remembering to replace the trigger adjust bit if you removed it from the stock earlier). Replace the stock screws.

This is like most Milbros in that it should be better. The addition of a 2nd pin for the cylinder sleeve/ trigger would help. I prefer the simple idea of the safety mechanism on this to the G25/27 etc set up, but guess the actual catch was very flimsy. The front stock screw holes do not seem to be deep enough for my liking, and maybe the original screws came loose and resulted in the stock breaking. I only fired 5 shots with it as it was getting dark, but it does seem accurate at short range. It was dieseling a little but this is normal when you have cleaned a gun out and used modern lubes as the old stuff take a while to burn off.