To get started you will need to get out those stacks of Rotten flesh you just have laying about. Once you have these you will need to make some Purification powder, which is shown below:
Once you have made the amount of powder you need for all of your Rotten flesh you can craft it with some rotten flesh like below:
This will produce purified Rotten Flesh, this is the first stage of turning it into leather, you now will need to make a new tool, This mod adds a hammer, which is crafted like this:
The hammer has 64 uses, and will break once over used, once you have this hammer you now craft the hammer and the purified rotten flesh like this:
This will give you purified & Flattened Rotten flesh, the hammer will take 1 damage per each rotten flesh, don’t worry there is only one more stage to go, now to use the purified and flattened rotten flesh in a furnace, like this:
Now you have your leather ready to make Book shelves, leather armor and everything else.