Lay your cardboard flat on a work surface. Mark these measurements on the cardboard and cut the strips. Then, mark a line up the center of the strips and fold them in half using the center line as a guide. Use a sharp utility knife or sharp scissors to cut your cardboard. Measure 3.2 cm (1.3 in) from the end of each strip and cut them to create tabs. Then, fold these tabs over each other and glue them together with hot glue. Folding the tabs will reinforce the ends of the cardboard strips. Don't cut these squares out of your strips! Use the rest of the cardboard you have. Then, cut the squares in half diagonally to create perfect triangles. Place the top end of each triangle on top of the fold line. Put glue on the sides of your triangles and put them in place along the strips. These triangles will provide support to your long cardboard strips. To make this easier, cut your big piece of cardboard so it's 56 cm (22 in) in length. Then measure and mark the 2.5 cm (0.98 in) segments along the cardboard. Cut along the lines and fold the pieces over to create a square tube. Measure from each end of the strip, mark lines across the segments, and cut along the lines. Fold the strips at each end to make tabs. Then, fold your square and tabs up and glue it all together. Place glue on your segments before you roll your square tube up. Using the rest of your cardboard, mark this rectangle with a pencil and cut it out. Measure and mark 5 2.5 cm (0.98 in) segments on the rectangle. Then, fold these segments to create a square tube that is approximately 2.5 cm (0.98 in) thick and 56 cm (22 in) long and glue it. Folding the cardboard like this will create sturdy legs for your easel. Make a mark at 3.8 cm (1.5 in) from the end on one of the legs. Flip it over so you can make a mark on the back side, then piece through both holes with a pencil. Place the other 2 legs right next to the pierced leg, mark holes at the same place, and push the pencil through these holes. Then, lay the 3 legs side by side and push the pencil all the way through to connect them.  The pencil is what holds the legs together. Stand the 3 legs up, then push 2 of the legs forward to be the main legs, and push the middle leg back to be the support leg. There's no need to glue the cross-member to the support leg as the support leg needs to be able to move. Mark your desired distance on each side leg and place some glue at that point. Put your cross-member square tube on top of the glue and let it dry for 20 minutes or so. At this point, you've connected all 3 legs and you've connected the cross-member to the 2 side legs. The string will provide additional support to your easel by preventing the support leg from sliding too far back.

Summary:
Cut 2 strips of cardboard, 56 cm (22 in) long by 6.4 cm (2.5 in) wide. Create 2 tabs and glue them together for support. Cut 4-8 squares of cardboard that are 3.2 cm (1.3 in) each. Cut a 13 cm (5.1 in) wide by 56 cm (22 in) long strip of cardboard. Make 2.5 cm (0.98 in) tabs on each end of your square tube. Measure and cut out a 13 cm (5.1 in) by 41 cm (16 in) rectangle. Pierce the cardboard to connect the 3 56 cm (22 in) long legs. Place the easel flat and glue the cross-member to the 2 side legs. Tie string around all 3 legs and stand your easel up.