Steam blocking sets the stitches with steam instead of a soak. The piece is pinned out to size, and a steamer is held right above it so the steam goes through the fabric and loosens the fibers. You never press down on it the way you would with an iron.
It is quick. A couple of minutes back and forth is usually enough, and the fabric should feel hot and only slightly damp when you are done. Leave it pinned until it has cooled.
Steam is the method that works on acrylic, which a soak does little for. Go gently there: too much heat flattens acrylic for good, so test on a swatch first.
1 Pin the fabric out
Start with the whole piece pinned out to size. It already looks better flat, but without the pins it would pucker back up.
2 Hold the steamer right above it
Hold the steamer right above the fabric. Do not press down on it like an iron. You want the steam to go through the fabric, which loosens the fibers and lets the stitches set.
3 Work back and forth
Go back and forth until the steam has gone all the way through, everywhere, nice and evenly. A thick fabric takes longer. Feel it: it will be hot and should be only slightly damp. A couple of minutes usually covers it.