I've done a bit of this before - covering wood with leather. We have 3mm thick leather here and the best way is, unfortunatly, to skive it down. A race taken along the two folds will help form a crease and stop the strength of the thick leather pulling out and 'springing' away from the glued surface. Prep, prep, prep, glue, clamp, clamp, clamp, wait, wait, wait.