There are three plates to choose from – they have different sized holes. Select the plate you want, depending on how fine a puree you want. Put it in with the rough side up, pointing up like a mountain. Put the end of the crank arm (under the spring and blade) into the hole in the middle of the plate, and spin the bar around so the ends are near the clips on the sides. Slip one of end of the bar into one clip, then press down on the other end of the bar and press it sideways a bit to guide it into the other clip.
To use the food mill, set it over a bowl (it rests on the handle and two supports). Put the food into the bowl of the food mill and start cranking. It should go pretty fast. The pulp will be pushed through the holes in the plate and the skin and seeds will remain behind. You’ll want to stop and empty that out once in a while.
If you want to try another disk, here’s how to change the disks:
The crank arm comes out of the middle of a skinny diamond-shaped bar that has a spring under it. Push down on one end of the bar to clear the clip on the side, and swing that end to one side of the clip, then release. Then you can easily remove the other end of the bar from the other clip. Then pull up on the red knob and the bar and blade will all come out. Then just push up from the bottom to remove the plate.