I used full extension slides that would carry a good load so I didn't have to worry about the shelf sagging when it was extended.

Centerline 100 lb. Full Extension Drawer Slides, Black Finish

I mounted the slides to the cabinet wall and took measurements to make the shelves.
To make the frame I started by measuring the opening and depth of the cabinet. Be sure to allow for anything that protrudes like hinges or recesses on doors. Except for the face of the shelf, subtract the width of the slides plus 1/4" to give a little extra room for dare I say "mistakes". For the face just subtract 1/4".
The frame is made with the sides and slides being inset in the face and the back going in between the sides.

Cut the front, back and sides to length.
Cut a dado in the front allowing for the with of the sides and the slide.

You could set up a dado blade but I used my favorite Freud General Purpose Blade.

Freud® 10'' x 40T Hi ATB Premier Fusion General Purpose Blade

I set up a spacer on my fence as a guide and just made several cuts to clean out the dado.
Next I cut the slots for the bottom of the shelves using a router with a spiral bit.

Spiral Mortising Router Bits

Lay out the frame and carefully measure to get the size for the bottom.
Cut the bottom and dry fit the assembly to make sure that it goes together as planned.

Glue the ends but let the bottom float in the dadoed groove.

Nail the ends with a brad nailer.

Porter Cable Pin Nailer Kit

Clamp and square the shelf by measuring from corner to corner making sure that the dimension is the same both ways.

These plastic squares work well also for squaring up your projects.

Clamp-It™ Assembly Square 6-Pc. Kit

I slid the shelves into the cabinet and they fit perfect!
Marilyn is now a much happier gal and it really makes getting at things at the back of the shelf easy.

That's about it. We hope you liked this project. If you build it and your friends ask where you got such a clever idea, please tell them that you got it at RunnerDuck.com.