Your cake needs to be as high as the bag and the fondant will stick to the buttercream crumb coat on the cake. Not too heave a crumb coat though or it will sag..
Color and roll gumpaste very thin and dry pieces on tissues to make it look like it was crushed.
Tissue: I use fondant and mix in tylose powder. Roll it thin, I use my pasta attachment on my kitchenaid. Use a pizza cutter and cut random squares and triangles and place on scrunched up foil to dry.
In advance I make fondant/tylose panels for the sides of my cake and allow to dry. I've heard you can make these and freeze them instead of drying them inadvance which I'll try in the future.
Make a 2 tier stacked cake so that your cake is 8" tall and whatever width you want. Attach the panels.
Here's a gift bag I made for a birthday.
Thank you both for your help.
Jacqui, your cake is absolutely gorgeous!
Thank you Soniabean! I hope we were helpful to you!