I am taking the 3rd course of the Wilton classes right now (finale cake tonight, wish me luck!). My instructor told us to dab a little piping gel on the sides of the cake (under the fondant) close to the bottom so the fondant will stick to the cake. I keep getting buttercream from my base icing in my piping gel, and then when I smooth the cake, it seems to ooze out the bottom and get on my hands making a sticky mess.
Do I just need more practice, am I doing something wrong, or is there a better way to "glue" the fondant to the buttercream?
Put the cake in the fridge for a little while to firm up your buttercream. Then just put the fondant over the buttercream. If your buttercream has crusted hard, sprinkle it with a bit of water first, otherwise you don't need anything.
I've never in my life used piping gel under fondant on a cake.
Like Texas_rose suggested, most people use water.
You can use many other things. Corn syrup, honey thinned in water, jams (no chunks) and many other things. Just don't put too much because it may dissolve the fondant.