1 plastic bag; scotch tape; a pair of scissors; 2 fairly straight twigs; yarn, string, or thread; feathers, ribbons or other tail decoration; 1 surly child wearing her brother's clothes (okay, child is optional)
Lay the plastic bag flat and cut off the bottom.
Spread out the bag so that there's no doubling of plastic at the edges.
Fold the bag in half and cut an elongated diamond shape. Use your longest twig as a rough guide for the tip to tip length. Try to leave the side corners still "connected" - in other words don't cut through the fold.
It should look roughly like this.
Lay your longest twig on top of the two layers of bag and tape at the top and bottom - don't tape the middle.
Lay the shorter twig across the long twig from side corner to side corner. Tape at the corners. To add some staying power, wrap the plastic bag around the twigs at each corner and tape on tightly.
Use your "string" to wrap around the two twigs where they meet in the center. You'll need about a 6 inch tail to wrap enough. In this picture I didn't have enough and had to redo. Wrap under and over crossing over diagonally.
Remember the bottom you cut from the bag. Oops, forgot to tell you to save it. Find it. Okay, now that you've found it, cut it open at one end only and spread out.
Firmly tape the feathers or whatever along it. Extra points for using this as a counting and pattern recognition lesson. Leave a longer "blank" space at one end for tying. Tie it around the bottom of the kite and twig, just double knot the plastic.
Realise you need a spool and rummage through your toy box. This lego block worked well for us. Cut your yarn, string, whatever so that it's approximately 3-4 times the length of your child's height (eyeball it). This seems to be the easiest length for them to handle while still providing maximum flying potential.
Have your ecstatic child pose with the finished product. Warn her that low wind means it probably won't fly no matter how hard she wishes. Realise that she'll think it's "flying" if you can just run up and down the driveway with it trailing behind slightly elevated in the air.