There are several ways to get a crackle effect with acrylic paint. This is an easy one using just glue and paint. I’ve never tried this one before and the effect is interesting.
This is actually a very old way to make crackle glazes for furniture. 1) I painted a wash of 2 acrylic colors crimson and pthalo green (use any colors), 2) I coated the small canvas with Elmers glue, thick and thin – let it set up for 5 min. or so (do not let it dry!), 3) Next, paint very lightly with some other color mix. I used crimson and black. This is hard to do as the glue is not dry so you need to have a fairly wet wash of paint and a light touch. The acrylic on top dries faster than the glue – so it cracks. Leave it overnight for the glue to dry. ( I prefer to use marble dust and gesso for this effect, actually.) The final result puckered a bit as I hit it with a drier. The glue does take a long time to dry. The glue is transparent when dry so the original color layer shows through.