Use a Silk Touch tool to break a grass block and pick it up
Find grass blocks in naturally generated areas such as plains, forests, and villages
Use a Silk Touch shovel or pickaxe on the grass block
Place the grass block where you want it
Let grass spread from nearby grass blocks onto dirt
Use bone meal on grass-covered dirt to grow more grass around it
In Creative mode, take grass blocks from the inventory
Use commands such as `/give` to obtain grass blocks in worlds with cheats enabled