Instructions
Travel to the in-game location where you would like to add a mob spawner.
Open the ̶0;Minecraft̶1; design console by pressing the ̶0;T̶1; key.
Type ̶0;//wand̶1; (without quotes here and throughout) and press the ̶0;Enter̶1; key. If the command is successful, you will be given a wooden axe.
Type ̶0;//give stone̶1; to create a stone block. Place the block where you want your mob spawner.
Click the stone block to set it as a starting point. Right-click the block to close selection -- you can set multiple blocks as spawners if desired.
Type ̶0;//set mobspawner|Skeleton̶1; and press ̶0;Enter̶1; to create a skeleton spawner. You can replace ̶0;Skeleton̶1; with any other enemy type, but you must capitalize the first letter.
Watch as your mobs spawn into the world.