When the pieces are clean, liberally apply the glue to all surfaces being fitted.
Working with PVC Pipe
From Stephen Hallo III, former About.com Guide
The fitting process starts by preparing all the surfaces to be glued. Cleaner or primer are used to do this. This will prep the surface for the glue and help enhance the chemical-fitting process that will take place. Brush the cleaner all around the fitting and pipe to remove all dirt and stains. A good way to ensure the pipes are clean enough is to remove the the lettering off the side of pipes. Be sure to prep all surfaces being fitted.
See More About
Explore Plumbing