DE in a sand filter???

[Rangeball]

Not to sound to ignorant but when you put the DE in the skimmer what setting is the filter on? If it is on filtering the DE would be pushed out into the pool.

You leave it set on filter. It catches at the top of the sand and does not return to the pool unless you have a filter problem.

Do you leave it on backwash? For how long?

You only backwash when you want to clean your filter. If you backwash after you've added DE, it rinses out and you have to re-add it.