Put the standard queen fitted sheet on the mattress upside down, pull it tight at all corners, pin up all the slack that you can near one end of the sheet. Sew up where you have pinned the sheet to make the sheet permanently shorter. You should have a flap of material hanging in the flat portion of your sheet, it will be on the bottom of sheet when sheet is put on mattress correctly. Then put the sheet on the bed correctly and it will fit your mattress.
Hope my instructions are understandable.