How do you make garlic wreaths to keep vampires away?

A couple of ways, the way to do it without cheating would be to gain Vampire Lore skill, that is where they learn it from. Make sure your sim has plenty of garlic in their inventory when they do start to make them.

The other would be in buy mode after you use the cheat bb.showhiddenobjects

Also, you can have your sim plant a whole bunch of garlic plants around their house, especially around the outside entryways. I had so many garlic plants in my sim's front yard, it even caused Vlad to turn away.