Spread the filler over the low spot as smooth as you can. The more you lay filler the better you will get at it, which will save you some sanding time. As you can see Vic spread the filler well beyond the low spot, this put enough material on the panel that you can sand off to make the area flat again. The bigger the low spot the larger the overage you will need to spread. Also, if the filler starts to harden while you are spreading it, just stop and let it dry. You can always knock it down with some sandpaper and try again if need be.