Once you have everything inlet, it doesn't matter when you put in the screws and pins. I generally drill for the lock nails, once the lock plate is in the wood and before I inlet the side plate so that I can use the nails to drive out the lock as I inlet it deeper. Also, I use the actual position of the lock nails to make the sideplate. Otherwise, it is really hard to get everything to line up. I make all my mounts and work from a blank so I have a slightly different procedure than someone who uses a kit or purchased parts.
I generally install the tang screw as soon as I can. When the design uses a wood screw, that is immediately after the tang is inlet. When using a machine screw, it is after the trigger is installed.
I generally wait on the barrel pins until I install the thimbles so that I can do them all at the same time since I setup the drill press especially for this operation. You really should install the pins in the forearm before you take the flat out of the sides of the forearm. There are a number of good reasons this which I learned the hard way.
I pin or screw the guard and butt piece as soon as I inlet them. The same goes for the toe plate and patchbox.
I have a build order list in the Tutorials. It is just mine though. Once the barrel is in, the order can vary depending on your experience level, external factors, and mood.