Rotten Mounds will randomly generate in Plains Biomes in a newly created world with this mod installed. If you come across one, it can be broken with your hand, or a tool, which will result in it dropping usually 1 piece of Rotten Flesh, and more rarely up to 4 pieces of Rotten Flesh in the latest version. If the plant doesn't break, its not yet fully grown, so give it a little longer to mature, however, it may break and return you the seed for you to start again.
After obtaining some Rotten Flesh, it is possible to create a growing area anywhere you like and craft Rotten Seeds to make a garden of Rotten Mounds. To plant a Rotten Seed, you MUST make your growing area out of Soul Sand. Rotten Seeds will only grow, and can only be planted on Soul Sand. The Soul Sand will not need to be hoe'd.
Rotton Mounds are slow growing, so be a little patient. If you are not a patient person, you can hurry the growing process with Bonemeal.
Here's how to craft a Rotten Seed