I've been wanting to make gladiator sandals for some doll and decided to make them for the skater doll. After all, this type of sandals are originally supposed to have flat soles.
The first photo shows the parts required: soles, zippers and straps. The straps are made of lace treated with sealer to keep its form. The insoles are pieces of leather with one layer of cardboard underneath. The white cardboard parts shown under the insoles did not make it to the final construction as it turned out that the soles need to be flexible or you can't get the shoes on the doll's feet.
Here you can see the zippers and straps glued in place.
And here is a back view. The problem with reqular sized zippers is that they don't open far enough. With high heel shoes, it is not a problem, but with flat feet, you need to be really careful with the length of the straps.
All that was left to do was gluing the leather outer soles in place. After this experiment, I'd say that buckles would be a better choice for this kind of shoes, although they would require more work both when making the shoes and when putting them on the doll's feet or taking off.