Can someone please tell me how to attach fondant to buttercream...
You drape it over? Try searching for "working with fondant" on youtube for some video tutorials. There are hundreds!
For smaller decorations If the buttercream has hardened I usually will moisten the back of the fondant ever so slightly with a washcloth then adhere it to the buttercream.
If it is a larger cake that has hardened I usually give a very light spritz of water onto the buttercream and that usually does the trick!
I usually just spray my cake with a light spritz of water and then lay it on. You can also use piping gel but water is cheaper You can also use water to attach fondant to your board as well if using wood. It works great.