There should be a handle you turn to the backwash postion. Push down on the handle and turn it to the right notch, let it flow a minute or so, and return it to regular filtering.
Do you have a sand or DE (diatomaceous earth - white powder) filter? Either way, the process is the same, but you may want to recharge the DE a little if you backwash it hard.
If you can find a make # model, i'll be able to explain it a lot better.
Are you using chlorine or a substitute stuff?
It's a sand filter, we think since we found a bag of it in the garage. The farthest we have gotten is checking the ph and chlorine and we did put in the chlorine tablets last night. Ph seemed pretty good. Tonight we are going to add shock and algaecide. However, we don't even know what valve is the backwash valve?
We need a crash course bad.