After you have bought the mutation you want, you plant the plant the mutation calls for around your planted zombie. There is a small chance that it will work, however you can get the mutation monolithe for 8 brains (mutations always sucessful).

For example if you want a potato head my fave XD you plant potatoes around a planted zombie,make sure its COMPLETELY SURROUND IT with the desired plant and make sure u bougth the mutation and if you can, fertilize the plants and there you have it XD

My fav is broco and venus fly trap , if u can't get a monolith get ur life force up it increases the chance of success

( What I did). So once you buy the mutation that you want, you'll see a gear in the top left corner ( I think you already know that.) Open the mutations file and it will tell you which crops you need to make the mutation. Let's say a tomato head zombie, then you plant a tomato next to any kind of zombie, and if you want to have a higher success, then you could get a mutation monolith. And when they are both ready to harvest, HARVEST THE ZOMBIE FIRST!!!!!! It won't work if you harvest the plant first. Then you should have the mutated zombie. Hope this helps:)

You place a crop that can be mutated plant it next to a zombie and once the crop is fully grown then harvest the zombie and it might be mutated.
also when you become a higher level you can unlock new mutations with gold and brains and if you have the mutant mongolith the all the mutations will be succesful.