Buy a length of PVC pipe that is 6 inches in diameter. It is up to you whether you wish to buy pipe that is threaded on one end. If the pipe is threaded, buy a threaded cap for your squirrel tube trap. If the PVC pipe is not threaded at one end, buy a plug.
Cut the pipe to 5 feet in length. There is more than one way to cut the pipe: either with a miter saw or with a back saw. The method is up to you. If you do use a miter saw, be sure to lower the blade very slowly onto the PVC pipe when you cut it in order to allow the blade to make purchase.
Decide which bait to use. This is a matter of trial and error. For some people, peanut butter is an excellent bait, for others, sunflower seeds, unsalted peanuts or commercial squirrel bait. According to Simmons Pest Management, the tube of the trap itself is "an attractant, leveraging the squirrel’s instinct to run through or hide in an enclosure." The combination of the bait you use and the PVC pipe, which forms the tube of your trap, should be an irresistible combination for any squirrel.
Secure the cap or plug on the PVC pipe and insert the bait of choice. Place the tube trap in a location where you know there's plenty of squirrel activity. Wherever this may be, make sure that you secure your trap so that it is at an angle with the end containing the bait at the bottom of the trap. Thanks to the length of the trap and the slipperiness of the pipe's surface, the squirrel will not be able to climb out of the pipe. Use rope or wire to secure the trap in place.
Check the squirrel tube trap often. If you do not wish to kill the squirrel, trap it so that you can relocate it. You can tip the tube of the trap down so the squirrel falls into a net or a burlap bag. Make sure you quickly secure the mouth of the bag or the net before the squirrel escapes. If you don't care to relocate the squirrel, you can check the trap once a week or more to dispose of the body.