The Volas Mattress Topper is meant to sit on the mattress you already have, so you can keep using the bedding you already own.
Place the topper on the bare mattress. Put the same fitted sheet over both layers. The duvet and pillows go back on as usual.
The listing does not give sheet-pocket depth or topper thickness. If your current sheet is already tight, smooth the quilted surface first so the sheet can settle.
Check the listed Volas Mattress Topper sizes if the sheet and mattress names do not match.