If you are plumbing the drain under a sink using a kit, the straight pipe may not be long enough to reach the waste drain in the wall. A drain kit is designed for a typical drain, and some drain situations are not typical. A straight pipe that is too long can be cut, but one that is too short must be extended. All the parts you need to extend a drain pipe can be purchased from any home improvement store.
Measure the distance needed for the pipe to make the connection. Measure the pipe and subtract its length from the length needed.
Cut a length of pipe about an inch shorter than the result from the previous step. The inch is to accommodate the length added by the coupling. Use a hacksaw to cut the pipe as square as you can.
Apply PVC primer to about two inches of the ends the pipes. The primer will clean the surface and help the cement bond. Apply the primer to the inside of both ends of the coupling.
Brush the ends of the pipes with PVC cement and insert them into the ends of the coupling. Hold the parts together for a few minutes until the cement has a chance to bond. Let the parts set for 10 to 15 more minutes to ensure the cement has bonded fully before installing the extended pipe.