For sand and grass elmer's works, the best in my opinion is wood glue, little bit stronger of a consistancy. If your trying this for the first time use Elmer's
There are a couple of good ways to get the sand on the base any container will do, I have a cookie tin for big stuff and a small canister for infantry bases.
What works best for me is;
-Finish assembling the model
-Put elmer's or wood glue on base
-cover base in sand, (flip the base over the canister your using get the excess of and use for later
) and get excess off the rim of the base.
-Prime (main reason is the primer locks the sand in place coupled with the elmer's or wood glue it will rarely come off!)
*Let glue completely dry (I usually wait 24hrs)*
-Paint
-Seal (if you do it!)
-Then apply static grass
I also make sure with model's that use slotta bases I cut a little square of paper to size to cover the gap between the model and the base, before putting sand on the base (if you do this make sure to let this completely dry or the little piece of paper could shift)
If you need any help with this I can show you, I'm usually at the store on Mondays