I have a pretty simple but perhaps silly question. When covering cakes with buttercream, how do you stack them? Once the cake is out of the fridge for a while, doesn't the buttercream become soft and stick to the cake board of the tier above?

Cakes will start to become soft after sitting out. Once you have iced each layer and it has dried/crusted and excess board has been trimmed, then remove from frig, sprinkle the bottom layer with PS and stack. When the layers are cold and crusted, stacking is a breeze. HTH.

Leave the supports that are in the lower tier sticking up about an inch or so - set the upper tier on there and let gravity take it down. Doing it this way your fingers don't get in the way and mess up the frosting.

You can also use shredded white chocolate or coconut to keep the frosting from sticking. (instead of p/s)