I didn't use a top sheet either. It's just one more thing to tuck in around them, or that could potentially get loose and be a hazard.

If you do use one, I would make sure that it can't get higher than bubs' armpits anyway, like you would with a blanket.

FWIW I was also totally unsure of how to tuck in my daughter before she arrived. This, and very many other things, was something that I learned in the hospital. If you are staying in hospital or wherever that there are midwives about, you'll be expert at it by the time you get home. So don't worry too much now about getting it right