I have tried in the past, with limited success and I guess the bag of skins grew and grew because I wasn't keen to try again. (I remember a horrible bright yellow colour).
The calendar says that this dye is 'substantive' which means that you don't need a mordant to stick the dye to the fibres. But if the mordant makes the dye faster (fade more slowly) then I reckoned it was worthwhile, so I soaked my wool in alum overnight. I'm not sure what the wool is. I think this is yarn I spun during last year's Spinzilla (or possibly the year before) and it's probably Shetland combed top.
After boiling my first batch of skins (you're not supposed to boil dyestuff generally, but Fran has noted somewhere that she achieved a nice deep colour after boiling some onion skins) I was amazed at the deep reddish-brown. But disappointed at the result. The yarn looked yukky-brown, and even less pleasant after rinsing.
It's a nice colour, maybe with a hint of the 'turmeric yellow' about it in places, but generally very tasteful.