From my understanding of reading the posts of those people who've added DE to their sand filter, they add a very small amount through the skimmer on top of the sand. IOW, the sand is in, then they add the DE. The amount would depend on the size of filter, I imagine. You're looking for a pressure change of just 1 psi, so go slow. I seem to remember 1/4 cup of DE to start with and keep an eye on the pressure gauge. How long this takes I don't know, I've never done it.

If the pressure goes up more than 1 psi, backwash and rinse. That'll flush the DE out so you can start again with a lesser amount.

As to your other post about pea gravel, have you tried a search on "pea gravel" and "laterals"?