here and on Vintage Homemaking here. The process is not that difficult but I will explain step by step how I made my wisteria decor.
Step one: Purchase various colors of tissue paper. I found it looked best when you had at least three colors on each wisteria "plant." I did mix up the colors and do a few with only two colors and even a few solid colors, but overall the ones with three colors looked the prettiest.
Tips: I found it was easiest to make all the petals first before I started threading any together. I got a few paper plates and filled each with different sizes and colors of petals for easy access.
Don't throw away scraps from the larger petals. The folded part that you cut off can be used to make really tiny petals. They will be a strange shape, but since they are so tiny when you go to pinch them to create the petal they end up looking just fine.