I also bought a large tube of sand acrylic paint and a couple of big cheap brushes. Everything else I needed I already had.
I already had a couple of tubes of coloured caluk (brown and sand). I emptied them out into a big bowl and added plenty of water and mixed it all into a watery mud about the consistency of thick gravy. I then smeared it onto the mat and worked it in with a large plastic paint scraper and left it to dry. It made some interesting patterns.
I then got out a large 4" brush and semi-dry brushed some yellow ocher.
And then I suddenly remembered to take photos.
I was pretty happy with the result and the texture that came through due to the fibers in the fabric.
I then gave the whole thing a light sprinkle of sand. I wasn't expecting it to stick but I wanted it to join in on the next application of paint. Then I let the ocher dry.
Then I did some heavy brushing of the acrylic sand with a 3" brush. It did pick up the sprinkled sand and helped with the overall texture.