I used to dress my girls in a thin bodysuit (think bonds easysuit material) the sleeping bag then pop a thin blanket over them.
I think each person is different though, some people don't use blankets others put them in only the sleeping bag and then use normal sheets & blankets.
My girls were hot sleepers so often i would come back and blankets were gone so it was great knowing they were still covered and warm.

I never really worried about the bit of material, i just put the girls in bed and it laid where it laid