Two thin plastic garbage bags
Sculptamold or Plaster
3D printed or other water resistant and strong rigid component to press the leather over
Heavy object for extended pressing (5-gallon bucket of water or paint, sandbag, yourself, etc.)
At least 8"x8" of 2mm thick leather
First thing- protect your workspace! I'm using a plastic garbage bag cut in half.
Step 1: Create Your Press Mold
In this case I've 3D printed the bottom half of my press mold and will be making the top using Sculptamold (a lightweight cellulose compound). Plaster could be used for the top of the press mold, if reinforced. It could also be 3D printed with the positive on one side and the negative on the other. The most important thing is that there is a positive side (the image you want to press) and an indentation on the other half so that your leather will be properly forced into the shape.
The mold needs to be low relief (no undercuts!) and not too large on your first try. This mold is 7"X7" and 1" tall.
I covered the 3D printed mold with a thin layer of plastic so that the Sculptamold would not bond with the bottom half of the mold.
Step 2: Mix the Sculptamold and Apply It to the Bottom Mold
Mix the Sculptamold or plaster as per the manufacturers instructions.
Press the Sculptamold onto the bottom half of the mold, making certain that it has pressed into the crevices. After the 1st layer has been pressed in (pink) but is still soft, I added a few strips of thick paper to create a support matrix for strength in the center of the mold. After I added more Sculptamold (blue/purple) to create a mostly flat top surface. This is important as the top of the mold will need to be pressed firmly into the bottom in order to form the leather properly.
Step 3: Cut Down the Leather
Make sure that you have about 1" of leather overhang on all sides of your object. This will be important if there is any binding of the leather and give you the ability to trim the piece to the correct shape later. My bottom mold has cylinders on each side to keep it aligned, but that is optional.
Step 4: Demold the Top
After the Sculptamold has completely cured (6-10 hours, depending on the thickness and the humidity of your environment) remove it from the bottom mold. You should have a fairly accurate imprint of the bottom mold. It doesn't have to be absolutely perfect but should be a generally accurate representation of the bottom half of your mold.
Step 5: Wet and Press the Leather
Next up you will need to wet the leather in warm water. Place the cut piece of leather into a bath of warm water and allow it to set for about 10 minutes to ensure that the leather is saturated.
Once the leather has been properly wet, press it between the top and bottom mold pieces and place a heavy object on top. You can also use clamps! I placed a piece of plastic between the leather and the top of my mold since the Sculptamold will absorb water.
Wait until the leather is dry before de-molding.
Step 6: Trim and Admire!
Depending on what you decide to do with the object, trim the edges and admire! It can be dyed, painted, and finished in whatever way feels right for you. I brushed the leather with gouache- not waterproof but it gives a nice matte finish.