We can make totally pure biomes that spawn only what we want, and where we want by digging out an area. Lay a layer of stone - or Crimstone - or Pearlstone across the area you want to farm in. Form the foundation. Once it's all dug up, you can change this at will by simply changing the block type on top of the stone.

Seeds can be used to create/spread biomes over Grass. These include: Grass Seeds, whose purpose is to convert dirt into Grass, helping to create more Forest or other biomes; Jungle Grass Seeds, which are used to create the Jungle; Mushroom Grass Seeds, which are used to create the Glowing Mushroom biome;