I can tell you how I did it on several projects. I have embossed a bit, but I know that there are many more members on here where embossing is their expertise, but I will share my ways. You decide.
One way that I have embossed is to take a piece of 3-4-5oz veg tan leather I will prepare my leather with water plus and allow it to soak in. I would then cut the outline of letters or subject with my swivel knife and carefully bevel the lines. I would then use my embossing tools to push up from the back (inside) careful to follow or stay within the lines. When I am done embossing, I lay the project down and re-bevel all of my lines again carefully. I would then use my modeling spoon to round off the letters to make them look round instead of square. When I am satisfied with the embossing, I will allow my project to dry a bit. Then I will turn the project over and put some contact cement in the area around the embossing. while this dries, I would take some poster board or mat board (to size) and put glue on it as well. Once the glue is dry, I will join both pieces (carefully) pressing down around the outside of the leather only (lightly). Then I would carefully re-bevel so the leather and backing make contact and keeping the shape.
My second way is to place pieces of leather the length and wide needed to fit in the cavity of the embossed leather areas. Leather dust also works if you want to make a oatmeal type mixture and do the same thing. From here you just proceed with joining the pieces together and go about the rest of the instructions.
Like I said, it isn't purdy, but it has worked for me on several projects.
Good luck