Janet gave you some excellent advice about the process. I would like to stress something she said because so many people seem to forget to do that, or not know:

RINSE after backwashing.

The rinse cycle should be about 30 seconds. It reverses the water flow inside the filter and packs the sand back down. If you don't do this you WILL get sand in your pool through the returns.

Be sure you have plenty of water in the pool before you begin a backwash/rinse cycle. It tends to take a considerable amount of water, depending on how dirty the filter is. I usually do it either right after a good rain when the pool is over-full anyway, or I run the hose while I'm cleaning the filter.

Don't skimp on the rinse cycle or you'll be sorry You can tell when it's done packing down the sand by how much/how little sand remains in the sight glass. A few grains means you're done. Too many to count? Keep rinsing.