This is actually very simple to do, just make sure you find a necklace that has lots of strings, you could even just bead your own. Also, if you unpick carefully, you get a bonus scarf from the bottom layer!
Get your long hippy skirt and rip off a bottom layer or two, depending on desired length.
Turn the skirt inside out and put it on, hitch it up like a dress and put on a belt. Now pin a line of how much excess material you want to get rid of at the seam and sew.
Put skirt/dress on right way out and wear your necklace. Arrange in a nice pattern then secure it to the dress by hand sewing it in place. (kinda tricky, you need a mirror)
Done! try not to get the necklace tangled, it's a pain :)