You will need a few DA/SA parts to add safety to a LEM gun, namely, the following:
1. DA/SA Hammer Axle
2. V5+6 (or V1+2) Detent Plate
3. Slide Plate
4. Slide Plate Compression Spring
5. Safety Lever
which are parts # 1, 2, 3, 4, and 6 in this
thread.
The mentioned video is for a gun that already has the above parts. The guy was just replacing the left-side safety lever with an ambi set.
What you have to do is swap the LEM hammer axle with the DA/SA hammer axle and that means removing all of of the hammer group parts (sear, catch, disconnector, hammer, etc.) You could get by with removing just a couple of parts but this short-cut is a little more complex than removing all of the parts.
The tutorial linked below shows you how to retain the safety when converting from a DA/SA --> LEM so you can read about it and follow the instructions loosely.
http://www.hkpro.com/forum/hk-reference-library/63083-usp-c-lem-safety-instructions.html