If you're hoping to make a more realistic looking cake, then you might want to use fondant. You can roll fondant into sheets that fit over top of your cake or use the fondant to mould shapes and characters. Fondant holds the shape or design of anything pressed into it, which makes it a good choice for creating a denim cake. Use the fondant over a cake cut into the shape of jeans or use it on a round cake and make your own denim design. With a little practice, your denim cake will look like the real thing.
Find a textured rolling pin from a cake supply store or cake decorating store. The rolling pins are designed specifically for use with fondant and come in different styles, but you want the basket weave texture. If you look at a regular pair of jeans, you'll notice that the pattern on the jeans looks similar to this style.
Sprinkle a small amount of flour on a flat surface and sprinkle a small amount on the textured rolling pin. The flour keeps the fondant from sticking to the rolling pin and the table. You'll also want to work in a relatively cool room, as the fondant might dry out if the temperature is too high.
Roll the fondant out into a flat shape, using a regular rolling pin. If you use the textured rolling pin right away, the design will stretch as the fondant gets wider. Use a regular rolling pin until you get the fondant into the size and shape you need.
Place the textured rolling pin on one edge of the fondant and gently move it across the surface. Press down lightly as you move the rolling pin, to press the basket weave pattern into the fondant. Then move the rolling pin to the opposite edge and roll it across the fondant. This creates a pattern that looks more like denim and less like a basket weave.
Transfer the fondant from the table to the cake by lightly wrapping it around the rolling pin. Then unroll the fondant directly onto the cake. Smooth the fondant down, being careful not to stretch the fondant and ruin your denim design.
Make the denim look more realistic by lightly painting white icing or food colouring into the patterned areas of the cake and then airbrushing in a shade of blue darker than your fondant. This highlights the design and makes it look more like real denim.