You can see the old slide sides on the bottom two drawers.
Since the cabinets walls sit in from the drawer openings, I bought the new slides and brackets that are used to attach the slide rail to the back wall of the cabinet. Unfortunately, I failed to realize that while the drawer, and therefore the slides, are 20", the cabinets are about 6 inches deeper. The brackets wouldn't work. It was time for plan B.
My next idea was to get some 1x2s and screw them into the side wall, which would put the slide perfectly flush with the edge of the drawer opening. I was quite please with my ingenious idea, and put the first one all together, put the drawer in, and realized that the drawers were 1/4 inch narrower than the opening, so they wouldn't fit in the rail on both sides. Time for plan C.
I went back to Lowe's and bought some simple wood shims. I cut them so that they would be 1/8th of an inch thick. By gluing the shim to the back of the 1x4 and then screwing through the shim, I was able to put the track where it needed to be. Finally it worked.
New slides screwed into the side walls.
Closer picture of new side rails
All of the drawers fit in well, except one. That one had clearly been rebuilt by someone at some previous time, and they didn't put the face on straight. I took the face off, straightened it out, glued it, re-drilled the pull holes and put two screws in the back, and it was close enough to good as new.
So that's one project finished! Time to find something else to do!