I used modeling clay for the outer soles, because the straps were so thick that this was the best way to deal with them. This clay is the sort that dries on its own and doesn't need to be baked. The reason for using that kind of clay is that the other parts of the shoes probably wouldn't like the heat and the soles need to be in place while drying to keep the correct shape.
The clay soles won't stick to the inner soles as you can see. Still, it is important to have them in place while shaping and drying them, so you get the indentations for the straps in correct places.
When the clay soles are entirely dry, you just glue them in place (PVA glue works well) and maybe sand them a little, if the surface isn't smooth enough. In this picture, you can also see the purpose of those flowers in the back straps. They cover the ends of the straps which have been glued together.
Finally, I painted the clay parts and the sides of the inner soles and applied a coat of sealer. The heels are made of polymer clay and I mixed the color myself. It's a bit lighter than the soles, but close enough.
