I'm getting ready to cover a cake in fondant that is already covered in ganache. What do I use to make the fondant stick to the hardened ganache? I know some people use apricot preserves but I don't have any and I can't go get any. Also, if I have to make something, how do I do it?

melted chocolate

damp the surface of the ganache with cooled boiled water - it'll stick!

some people use sugar syrup and also piping gel smeared thinly over the ganache.

I always use simple syrup.

If you're talking about covering the entire cake just use simple syrup. Equal parts sugar and water, boil to dissolve the sugar and let cool. Then brush onto the fondant and cover the cake.

You can also use piping gel. If I was attached just accents I would use piping gel so I don't have to turn on the stove.