Anyone who has taken apart one of these things knows they are the devil to put back together. All pins both exit and enter through the left side of the pistol. Colt's old manual simply says, reassemble in reverse order. Yeah, right.
The trigger with bar, the magazine release and the sear spring are pretty self explanatory but problems soon develop.
For instance, once you finally pin the sear and disconnector in place, the little sear will spin in the gun, but pointing the frame up and pushing the bottom of the sear through the magazine well opening with a pin will position it properly. Just remember the hooks should point up inside the frame, not down through the magazine opening.
The downwards pointing hooks on the grip safety will be engaging the hooks on the sear, but only when everything is just right. The short end of the mainspring faces to the front, not the back of the gun. When you start to place the grip safety on is when the real fun begins. The hammer and the safety will go in after (and only after) that, but before (and only before) the grip is pinned in place. Your problem is the grip safety (which hopefully by now have engaged the sears hooks with a quiet little 'snick') has to be held in a semi-compressed position until after the hammer and the safety (which is also the hammer pin) are in place, and only then fully compressed so the grip safety can be pinned.
However, you are holding the grip safety in against the pressure of both the sear spring and the mainspring. Don't be too surprised if your hand begins to cramp in the next few minutes. This is normal.
While holding the grip tightly in one hand you forcefully insert the hammer (in hammer down position) with the other hand so that the roller of the hammer slips between the back of the frame and the stub of the partially compressed leaf mainspring, then with your third hand you line up the holes so you can insert your slave pin. Start to see the problem?
It can be done, and the U tube video shows a smith doing it, but notice how at a critical point, the gun leaves the view screen and there is a screen refresh when it comes back.
Old time Colt approved Smiths had a little jig which held the gun grip in for them thereby freeing up one of their hands. I haven't seen one of those jigs in forty years. My discovery? Scotch tape. Duct tape is good too. Position the grip, engaging the sear in the process, then while holding the grip in to the barely compressed position, stick it in place with the tape. 3 or 4 layers tight around the grip safety and the frame should be more than enough to hold it for a few hours. Leave the pin hole in the bottom exposed. You will be needing it soon.
On to the hammer. If your hammer is anything like my gun's hammer the holes will never match up so neatly that a slave pin of the proper diameter can quickly be inserted without any difficulty. What I find is the hammer will always initially be a little bit off to the left, to the right, or up or down. A slightly thinner slave pin can, in theory, be inserted into the hole from the right and with a little leverage move the hammer, left, right, up or down against the pressure of the mainspring as needed until the line up of the holes is perfect. Good theory. Sadly however, in my universe, the thinner slave pin bends but the hammer doesn't move.
My second great discovery? On early 1,2 and early 3 generation 1903 and 1908s the diameter of the safety pin is only a hair more than the diameter of a common 6D roofing nail.
Once you can see daylight through the right side when the hammer is almost in position, insert the pointy (use a smooth one, sand down any ridges or burrs if need be) nail into that space. Push. The nail moves the hammer perfectly. Now replace the slaved nail pin with the real pin (the safety, late gen 3 to 5 have keyed openings so also remember to line up the keyhole on the safety with the one on the frame) from the left side. Wiggle the nail a little if needed. The hole alignment should now be perfect and the safety just drops in. Using the nail it all takes about 20 seconds.
Now squeeze the grip tight. Insert the grip pin from the left side. Remove the scotch tape. The gun is now ready for the grips and the slide.