Embossing leather requires a tooling or other soft leather. Thicker, stiffer leather will not properly take to the stenciling, and you won't get the clear image you get with soft leather. Start by laying out your leather on a table. Dampen some rags and ring out excess water, then lay them on the leather to soften it for the embossing.
Choose your template and lay it upside down on a wooden board, then nail it in place. Take the rags from the leather and wipe up any excess water. Lay down a towel on the leather and use an iron set on high steam to help further soften the leather. Once this is completed, take the leather and lay it face down on the template, with the back (wrong) side facing up towards you.
Place a piece of Styrofoam over the leather and then weight it down with heavy items such as bricks or a stack of books. Allow the leather and weights to sit for six hours undisturbed. Remove the weight and the foam, leaving the leather in place. Check to make sure all of the edges of the design are clear. If they aren't, tap the back of the leather with a rubber mallet. When you're satisfied with your design, remove the leather and lay somewhere to dry.