You will need one oblong or square cake and one round cake. Alternatively, you can use two loaf cakes instead of the oblong or square cake.
Cut the square or oblong shape cake in half to make two long pieces and remove the centre piece of the circular cake. You can do this by using a glass to trace a centre piece which you can remove.
Resource: See cutting a zero cake
Use the edge of the round cake as a guide to cut a piece from the bottom of one of the long pieces of cake.
Once you have the top piece snuggly fitting against the round part, cut a section out beginning at the bottom edge of the long piece as shown in this picture.