Purchase the PVC pieces at your local hardware store. In some areas, the four-way fittings are difficult to find. They can be purchased online via agility training course equipment providers or other specialty PVC fitting stores. Have the store cut each piece to the length you need it, or purchase a PVC cutting tool. They're easy to use and do not require electricity. Be sure that you're using the same diameter of PVC in all of your parts. For sturdiness, 1-1.5 inches are recommended.
Lay out all of the pieces of PVC that you will be using. Assembly is easy; simply press the tubing into the fittings. You can use hot glue for a sturdy temporary adhesive, but for permanent sturdiness, use PVC adhesive (flux).
Assemble the base first. Affix two four-way joints to the bottom of a 4-foot length of PVC. These joints will be the bottom corners. Attach two elbow joints at opposite corners of each joint to secure the four wheels for stability and portability. If you don't want rolling backdrops, if your stage has a hard floor, choose PVC rubber end pieces instead.
Place the 8-foot side pieces into the tops of the four-way joints at the base to form the sides of the backdrop. Affix the elbow joints to the tops, and install the last piece of PVC to close the top of the frame.
Unfold the black fabric. Be sure it isn't reflective or it can interfere with stage lighting. Felt is very nice for this purpose because you can use additional pieces of felt to create removable trees, furniture or other things you might want in your background.
Lay the fabric over the top of the frame so that it is equal in length over both sides. Use the hot glue gun to affix the fabric to the PVC frame and completely cover the PVC. Be sure to leave a "skirt" of fabric over the base so the audience doesn't see the base of the backdrop.
Continue tucking and gluing the fabric to the base until it is completely covered. Trim excess fabric if necessary.