The best way to slice whole apples would be to use a mandoline. Set it to the level of thickness you would like and start shaving. You’ll find that the slices will come out evenly. If you have good knife skills, you can slice them by hand. It is a bit tricky because the apple is round, and it can’t lay flat on the chopping board, so it’s susceptible to movement. So do be careful if you’re going to cut freehand.