Lay both wings flat as shown and tie them to the body tightly using stretch string.
At any point you can tie a string to the center hole of the top bracket. (this string does not need to stretch and will tie to a ceiling or high place.)
The same goes for the bottom hole on the body of the bird. At the end of this string you want to tie up something with a little weight to hold the body down and help it bounce up and down, and in consequence flap.
Finally the most tricky part is to run the stretchy string through the two wing holes and up into each end of the main top bracket. Getting each wing string even can be a bit of trial and error.