Every once in a while you get a piece of furniture that either have a missing runner(s) or badly deformed runners. This can cause future problems if it is not taken care of when you are rehabbing the piece. Plus if you are painting it, it can ruin your finish after just a few uses of the drawer and that I can't abide after all the hard work I put into painting furniture.
So what do you do? This is a great tip and super easy to do.
At any craft store or hardware store you can find dowels in various diameters.
They also come in squares as well.
Just one of my drawers on my green dresser had a missing runner.
I cut it into two pieces that ran the depth of the drawer and then using wood glue, glued in place. Space them side-by-side, about 1/8" apart.
Use clamps at both ends and in the middle until dry. Once dry use small brads to further secure it.
I then usually use soap or candle wax to lubricate the runners and the runner in the frame to make it run really smoothly. Brand new runner for about $1.00.