Step 1: Supplies
Sturdy material- I used an outdoor canvas, which can be found in the home section of fabric stores.
PVC pipe- I used four 6" posts and eight 30" side pieces, but will add more 6" sections for reinforcement later. You may want to use more vertical posts upfront so you don't have to adjust later, in which case your side pieces would be shorter with more pieces (example: if you add one post to the middle of each side, you will need eight 6" posts and sixteen 15" side pieces to be the same size as the one I made).
PVC joints- I used eight 3-way joints for the corners. If you're putting posts in the middle of the sides, you'll need two T-shaped joints for each middle post you add. The 3-way joints they had where I bought my supplies had two joints that fit the PVC pipes & one that had grooves for screws, so I had to get a piece that screwed into the third section to fit the pipe (see photo).
PVC cement- To secure the pipes
Bungee cords- I initially had two 6' bungee cords that I was going to weave drum-style around the frame and through the fabric. We'll talk about that later. I ended up using 22 individual 8" bungees instead.
Scissors- Heavy duty enough to cut through your fabric. If you have a grommet tool to use instead, that would cut down on a few steps.
Step 2: Frame: Dry Run
(If you're using the T-shaped joints and extra posts to reinforce the sides, connect the joints with the side pieces to make a longer side piece before connecting with the corner joints.)
Start with a 3-way joint and set it flat so that two openings are parallel with the ground and the third is facing up. If you have to use the extra screw-in pieces like I did to make the third side fit, make sure that is the end facing up on each section so your sides will be even. Attach a side piece to each of the two openings running parallel with the ground. At the end of each side piece, attach another 3-way joint, then another side piece, until you have a flat square with four openings facing up (see photo 1).
Connect your four (or eight, if you're using more) post pieces to the open joints facing up. Attach the top 3-way joints to the corner pieces as you did with the bottom pieces (screwed-in piece facing the short pipe). Connect the top 3-way pipes to each other with the side pieces as you did for the bottom. Do this on all four sides. You should now have a complete frame.
Step 3: Frame: Cement It!
Before taking apart your frame, you may want to mark where your pipes and joints fit together so it's easier to ensure accuracy when you're cementing them together. Just draw a straight line from the joint onto the pipe about 1" long so you'll be able to line the pieces up faster.
Take your frame apart and piece it back together using the cement (follow the instructions on the cement you're using). You'll want to work both quickly and accurately on this step, as letting a piece sit for too long in the wrong position can cause the cement to set and ruin the entire frame.
Once your frame is cemented together, leave it outside to dry. I left mine overnight, although you may not need to leave it for as long.
Step 4: Create the Jumping Surface
Take your heavy material and fold in half to make a double-layer mat. Cut it to the same square size as your frame (32" square in my case). With right sides together, sew around all four edges (including the folded edge), making approximately a 3" wide hem. The hem can be smaller, but I made mine this wide so that later when I made holes in the fabric, the holes would go through all four layers and be reinforced for attaching the fabric to the frame. Be sure to leave an opening in the hem so that you can pull the fabric right side out when you're finished sewing it together.
Pull the fabric right side out through the hole you left. Sew around all four edges again, this time about 1/4" from the edge. This seam will help keep the extra layers in place on the inside, as well as close up the edge you pulled the fabric through. You should now have a flat fabric square about 3" smaller on each side than your frame (in my case, approximately 29" square).
Step 5: Make Some Holes
If you don't have a grommet tool, create holes around the edge of your fabric approximately 2" from the edge, 4-5" apart. I did this just by cutting X shapes around the edge; there's no need to cut away the extra material to make a hole, unless you don't like to look of the X-shaped holes.
To prevent the holes from tearing when attached to the frame, sew a box or circle around them for reinforcement. This doesn't have to be perfect (see photos), just sturdy enough to prevent tears. Do this individually for each hole.
Step 6: Attach Mat to Frame
I chose to use short (eight inch) bungees to attach the mat to the frame. My original idea was to use several long bungee cords and weave the mat onto the frame (similar to weaving a drum skin to the body), but I ran into several issues with this method. For one, it takes a LOT more bungee material that way, and it caused the project to be economically unfeasible if I wanted it to cost less than buying a preconstructed trampoline. Second, it is far more difficult to adjust the position of the mat as you go if you're weaving it on with bungee material, and you could end up with a lopsided trampoline.
Anyway, with the short bungees you should be able to attach your mat fairly easily. Start by attaching the mat in one corner by looping one bungee through the hole in the mat and around the edge pipe. Secure with a knot. Since my bungees are made to be hooked around a ball on one end, I hand sewed the bungees together with fishing line where they met to be sure that they wouldn't come flying off later. You may want to hand sew your knots together with fishing line as well for added security.
Now that you have the first corner attached, repeat the step with the corner diagonal from the first, then with the other two corners. Your mat may look odd at first, but it should get taut as more bungees are attached. Continue attaching the mat with the bungees, alternating sides, until each hole is connected to the pipe with a bungee.
Step 7: Strength Test and JUMP!
If you built a frame that needs extra supports, you don't have to start over completely! Just detach your mat and cut the pipes in the middle on each side, then reconnect with the T shaped PVC joints and extra support posts mentioned earlier. If you add more than one support post on each side, you may have to make a larger mat for the frame, depending on the bungees you used.
Once you've sufficiently weight tested your trampoline, you're ready to jump! Since this frame is lightweight, it is best if you attach the base to the ground in some manner (duct tape would be acceptable for most surfaces). This will prevent the frame from shifting as it's used.
As always when using a trampoline, exercise caution and good judgment, and don't allow children to use unsupervised. Be safe and have fun!
***UPDATE: After having my nephew jump on this frame, I'm definitely going to reinforce the sides before he gets any bigger. You can still use this basic model for adults, but I would recommend swapping the PVC material with metal pipes for safety purposes. You can buy threaded, pre-cut pipes and joints at many hardware stores if you (like myself) do not have the tools to cut and solder metal.