This ain't no build thread!
To reinstall the keepers, I use a slightly magnetic electronics flat head screwdriver. I put a dab of assembly grease on the concave face of the keeper, sometimes the blade of the screwdriver blade for extra adhesion. Compress the valve assembly to expose the valve stem relief, place the keepers on. You can see and feel when keepers are in the relief, they won’t slide down. Once in place, release the compressor cleanly, in one quick motion. Look to see that the retainer seated around the keepers. If it doesn’t (and it’s rare that it won’t), decompress with the tool and check the keeper position again. Sometimes just cycling the compressor tool again will seat everything just fine.
Head work is repetitious, nuanced, but I find it strangely satisfying.