I'm making a cross cake in a few days and will be covering with mmf with a leaf impression. I've only done a few fondant covered cakes and I've always used a cake board the same size as my cake, covered it and placed on a bigger board. Will I need to make a cross shaped board to go under my cross cake before I cover it, or could I cover it on a rectangular board? Any other tips for covering the Wilton 2 cake mix cross cake would be appreciated. Thanks!

I personally would just put it on the rectangular board..
Another tip I would suggest (as hard as it is) would be to focus on the corners first. If you want a smooth covered cross without any creases, definitely do the corners first. Once you get them perfect, then to the sides.
Good luck!!