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Tonight, I had to fit a new thumb safety on one of my 1911, so I figure I would post of some steps on how to fit one.
Tools:
Flat diamond file.
Round diamond file.
Oval or half-circle diamond file.
Masking or duck tape.
Dummy round.
This should apply for both the single-sided and ambi.
Here's what the new TS will look like, and the flat diamond file.
A close up of the blocking lug.
1) Remove the original TS, if it's a stock gun. If building a new gun, this will be your TS.
2) Remove the BTGS.
3) Assemble the ignition and everything back together. This way you can put proper pressure on the sear, and cock the hammer. You'll want to have the slide on the gun, just incase the hammer does fall, so it's not to hit the frame.
Here's the virgin TS inserted into the gun. Note the blocking lug still have sufficient metal on there and it can't go pass the sear.
4) move the TS back and forth, rubbing the blocking lug against the sear. This will mar up lug, giving you a reference point to start filing. If you're doing a stainless TS, mark the lug up with a black Sharpie marker.
5) Using a flat diamond file, you'll want to file slowly, keeping the file parallel to the locking lug. The flat surfacing, you're trying to file into the lug, needs to be perpendicular to the top surface of the locking lug. Remember, to go slow. Check your progress by inserting the TS back into the frame. It's always easier to remove metal, than it is to put it back on.
6) Once done, you'll be able to fully seat the TS into the frame, while blocking the sear.
7) Test function the TS to make sure it's blocking the sear. Click it on and pull the trigger, making sure the hammer doesn't move. Give this a few tries. With the TS on, and hammer back, put the gun close to your ear, and cock the hammer back further. Listen for a click. If you hear a click, this means the sear had moved slightly out from the hammer hook, and just reset itself. This is a problem, and you don't want to hear that click. This means that you had taken a little too much off the sear block.
If a click is hear, you have two options:
a) peen the sear block and "push" blocking surface forward, thus filling in the void. Place the TS flat (outside surface) on a hard surface, like a vice (use tape to avoid marring the finish). Hit the side of the sear block, which now faces the ceiling, a few times w/ a hammer. Insert back into the gun and perform function check.
b) start with a new TS and file again.
This should be it for testing the TS and sear engagement.
The second part is to adjust the TS for positive disengagement click. This fitting does two things: a) it helps the TS to stay put, when engaged, avoiding accidental disengagement, and b) provides a nice "click" when you disengage the TS.
With the TS engaged, you'll notice the plunger pin making contact w/ the TS's detent area. For a nice positive seating of the plunger pin, that surface must have a deep concave, and a sharp corner for it to break, as the TS swings down and disengage.
1) Use a round diamond file, you can start to file at that surface, increasing the concave radius. Make sure you leave a nice sharp corner at the end. Both the concave and corner are what locks the TS in place, helping it to stay "on." Use the masking tape to avoid scratching surrounding areas.
2) insert and test the TS until the desire thumb pressure is needed to disengage the TS. You'll want some pressure to move that TS w/o it being too easy, thus avoiding those accidental movement of the TS to "off." Increase the concave surface, until that pressure is reached.
3) When done, clean up the area by smoothing out all filing marks. Degrease/oil the surface and apply some quick bluing, if needed. For stainless, you'll might want to reblast the TS for proper matching shine/matte texture.
just needed some filing on the area that the safety plunger detent contacts the thumb safety. Smooth as butter now! 
