1. drive the roll pin out and remove the existing latch
2. look for spring that jumped out when you did 1.
3. curse. curse. curse again.
4. find spring. now look for the roll pin, which mysteriously disappeared while doing 2.
5. see 3. repeat.
6. tap the roll pin till it just shows a little (maybe a 64th or so on the inside of the charging handle.
7. fit the spring into its hole in the handle.
8. press the tac latch in and try to line up its hole with the roll pin.
9. whole thing falls apart. see 4,5. repeat 8
10. when you "feel" that the hole is matched up with the end of the roll pin, GENTLY tap the roll pin thru.
11. scream a primal victory scream.
12. put your AR (or M-16) back together and rush down to the range to try out your new tac latch.
I did THREE of those little buggers! Can you tell?