I use thick shoe leather a lot for bow handles. I wet the leather until flexible, remove excess water and glue it down with TBIII. Once dry(at least over night) you can trim the excess with a sharp knife or razor.
Once you are sure it is completely dry, saturate with super glue and let that dry. You can now sand it smooth. You may want to do this last step a few times to get a smooth surface. I also make tip overlays the same way.
here are a few pics of the process. Soak the leather until soft in warm water...
...size both the leather and handle area of the bow with TBIII...
I then wrap with strips of old be sheet. Big rubber bands also work well...
...once dry you can shape with sharp blade and in this pic a wood rasp. I usually add two layers to the handle to fill it out...
You can see the two layers of leather and the beginning of the super glue...
here is the finished laminated leather handle riser...