Making it hard like a board is usually done at the tannery. But you can make veg tanned leather pretty hard by soaking it in very very hot water, and then drying it in hot sun. you can repeat if necessary. Drying in an oven also will work, but that becomes very tricky…
If you’d like to stamp or carve the leather, you’ll want do that part first. After you’re done, you’ll need to let it dry. Then use the hot water. I think that you need to experiment first, and I think that you’ll need to allow for some shrinkage. You may have to try drying it in an oven, but you’ll have to use low temperature, and watch the results carefully, otherwise the leather can become brittle.