I’ve been making these for years, starting when my own boys were young.
I usually use clear plastic (we have a huge roll of it which we use to line our backyard rink with). It’s a really sturdy plastic and super slippery. You can pick up a roll at most hardware stores. I didn’t haul out the big roll today because I had this large tarp in my garden shed, and I figured it would do the job just fine. And it did. :)
I spread the tarp out over a small slope in the yard, hung the garden hose up over the clothes-line, and chose a light spray setting. I used that little clip thingy on the hose nozzle to hold the trigger down so the hose would stay on. Then I drizzled some cheap shampoo over the tarp to make it slippery. You don’t want to miss this step. The kids won’t be able to really slide without it. Just pick up a bottle of shampoo or dish detergent at the dollar store. You don’t want to spend much because you’re going to use a lot of it keeping the tarp slippery. We went through the whole bottle today, but I got it for a dollar so no biggie.