Do you cut them out and let them dry or put them directly on the cake?
If it's something that's going to lay flat on the cake, I cut them out and put them directly on the cake. This is especially important if I need the decorations to follow the form of the cake (like looping over an edge or something). If I need them to have a little shape of their own, I let them dry the way I want them and then put them on the cake. If I'm putting fresh fondant on a freshly covered cake, I use tylose glue (1/2 cup of water mixed with 1/2 teaspoon of tylose/CMC and left overnight to thicken) to stick them on with. If either the cake or the decorations (or both) are dry, I use royal icing to hold them on.
Sorry, I didn't read the first question correctly. I roll it to about a quarter of an inch to cover a cake and different thicknesses for decorations - again, it depends on what the decoration is. Tissue paper thin for flowers on up to over a quarter of an inch for heftier things (I just made a baby bassinet that was pretty sturdy). Some people prefer to roll it thinner for covering cakes, though.