This instructable was inspired by the first thing I ever made in a woodshop, a very similar pen-holder project. That project was designed by my shop instructor and to be created using only basic hand ...
Using a bandsaw, follow your markings and cut out the flip-flop. Try to trace along your markings as closely as possible without digging into the form. Take your time and be meticulous about these cuts.
(If you do not own a band saw, you may use a reciprocating saw.
If you do not own a reciprocating saw, further trim the template and then use a disk sander grind away the excess)
I chose to use a band saw because it provides a level surface and it is easy to manipulate the position of the wood
After you have finished cutting out the flip-flop, quickly compare it with the original and make sure there are no large areas you forgot to remove. It is much easier to remove them with the saw now than it will be when sanding later.