Piss easy but the leaves have to have THC crystals otherwise it is useless.
Dry the leaves then take out all the twiggs. Then grind them in a mixer. Then get one of these
But make sure it has extremely small holes.
Work the dry leaves on it pretty well, make sure all the crystals fall because you really want them all, don't you?
Then get some butter, melt in a pot, add the cannabis crystals, here it is tricky, the mix should not become too thick, it has to stay "buttery". If you have more crystals just melt more butter in another pan and add them. You will notice, the melted butter has to look like melted butter only green in colour. Keep on stirring until they look infused, you will notice yourself. Usually it is when the texture stops changing.
Then get a plastic container (preferrably with a lock), pour the melted butter in it and put it in the freezer. The melted butter becomes butter after a while (safest bet is to leave it 24 hours.
Now you have cannabutter, you can go look for brownies recipes or milk shakes that have butter as ingredient. Enjoy.
WARNING: Your house will smell for a day or two.