One of the biggest problems for beginners is knowing how much to sand to remove all the marks created by jointers and planers. Here’s a trick you can use to indicate when you have sanded enough.
Draw some pencil marks on the wood, then sand until these marks are gone. You could even do this a second time to be extra sure.
Use these pencil marks only when sanding with your coarsest grit sandpaper. Don’t draw the marks with the finer grits. Very little sanding is needed with the finer grits to remove the coarser grit scratches. It’s most efficient to sand out all the problems with just the coarsest grit sandpaper.