Mix colors into even balls of clay, graduating from light to dark

Here I mixed white, orange, and yellow

They should look a bit like this, in the colors you want

Flatten the balls into even disks. Stack them to make sure they're even (or use a cookie cutter)

Roll out white clay for the filling, and place a disk on the rolled out clay. Cut around the disk (or use a cookie cutter).

Repeat until you have enough for in between each layer, and one for the top and bottom

Roll out another strip of white clay, and wrap around the cake. Smooth out the sides. Be careful not to distort the bottom and top layers.

Trace out where you will cut (so you can smooth it out if you make a mistake)

Cut your slices

Insert a pin to make it into a charm. Texture the cake portions with a needle.

You can bake now, so you don't risk ruining the texture, OR go on to the next step

Decorate

Bake according to packaging instructions on the clay