Since I have already made this same repair twice (once on my own appliance and once on a neighbor's), I'm getting pretty adept at it! Here are the steps:
1. Remove microwave oven door.
2. Using a putty knife, gently pry out the black plastic outer bezel.
3. Remove two visible screws near the latches that secure the inner prongy metal frame.
4. Using a putty knife and/or screw driver, lever up the metal frame by releasing the small tabs on one or two sides - (this is the trickiest part of the whole operation).
5. Unscrew the now visible screw holding the broken Spacer and replace the Spacer, making sure to affix the handle to both Spacers before you tighten the new Spacer in place.
6. Replace the metal frame - it should snap back in fairly easily if you stretch the outer frame slightly.
7. Screw the retaining screws back in near the latches - it'll be hard to get the screws into the holes because you have to get them between the metal prongs of the frame, so use the following method: take the plastic bag the Spacer came in and poke a screw through it. Insert the screw driver into the screw and hold the plastic bag so that you're applying tension that will keep the screw on the end of the screw driver as you lower the screw between the prongs and into its hole.
8. Replace the plastic bezel by gently snapping it back in place.
9. Remember these steps, because you're likely going to have to do this all over agin in a few months when one of the Spacers breaks again. I think the curve of the handle is nor designed or manufactured quite correctly, which puts undue strain on the cheap plastic Spacers, which results in them snapping easily. To mitigate this, make sure to grab the middle section of the handle when opening or closing the door, rather than pulling it open near either of the Spacers. This will help to distribute the forces affecting the Spacers.