need a flat sanding block, or sometimes called long block, that way you can block it down. You sand in an x pattern, like top left to bottom right then top right to bottom left, once you see some metal showing on either side or one side that's your high spots, you can then fill in th elows, block it again until it comes out nice and flat.
Mostly what you do is once the filler is on and you get most of it sanded down you use guide coat, which is like a powder, they have rub on and spray on, this leaves a misted black, kinda half sprayed look, you block this with a finer grit like say 180 or something 150 220 etc. You can then see when the guide coat comes off the highs and lows, at this stage you can refill with filler, maybe glazing putty since it's not as thick, or even high build primer, then repeat until it's coming out flat and smooth.
