Okay here's how you do it, and remember take your time or plan on having a spare selector for this mod.
I have a TGS steel selector in case I want to convert the gun back to an M14.
You'll need the following items:
-Donor Selector Assembly
-Epoxy (I recommend JB weld)
-M21 Dummy Selector Nub (http://www.sparrowhawkm14.com/id9.html)
So what you'll do first is to remove the entire selector assembly, which comes off with two screws. Using a dremel (I went for a fine bladed hack saw), you'll trim off the paddle (the part that lets you rotate from SA/FA) where the column meets said paddle. Don't trim it ANY shorter.
So you'll leave this alone for a few steps.
The Sparrowhawk ECO kit is riveted to the bracket that holds the selector nub on, so using a drill press you'll need to drill out the rivet. Once the parts are separated you'll REQUIRE the use of a metal vice to hold things in order. Start with a small bit (same one you used to remove the rivet). Going slowly you want to reach a depth of about 1/8th inch deep, it's steel so lubricate the bit. Then using about 2 bits you'll progressively widen the hole (I'll update this with the exact diameter in the morning) so it's equal to the same diameter of the selector assembly you modified a while ago.
So the way the M14 selector works is a tab that is rotated from the 9 'o clock position to the 3 'o clock position. By default SEMI is 9 'o clock. Remember this when you re-assemble.
If you took your time and did it right the two parts should engage tightly without any adhesive. But you want it to stay on regardless, so you'll put some JB Weld on the inside of the hole your bored out.
Check the fit, and then put the selector nub on to the installed selector assembly. The assembly needs to be set to semi before you attach the nub, because when you install it you'll want to make sure that the flat side of the m21 selector points up at the 1 'o clock position (reference my pics for the guide on angle).
Let it set for 24 hours to set up. You'll have two choices to make, you can STILL select full auto by turning the nub (it's not as easy anymore). Or you can permanently set it to semi-auto by either epoxying the sear on the back side of the connector to the semi-position (I opted to tac weld, via a friend's welding torch, it so it can NEVER move, it's your choice).
The end result is what you see in my photos. Overall the mod will cost you about 35 bucks, looks great, and doesn't look funny when you call your gun an M21.