Switch to your large brush to fili in the center of the shape with solid black.
Use your smallest brush and black paint to paint large cracks around the outs:de of the opening. Make some thick and some thin. some short and some longer so that they don't look too much alike.
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To make the smaller cracks, you can use a fine-tip black marker or a pamt pen. Or mix a little water with some black paint and use the tip of your smali brush to paint thin crooked lines that fan out from the ends of the cracks. some with two lines, some with morę.