That's a pretty ambitious costume for the first try ^_^;;;
I would advise simplifying it, honestly. You'll regret it if you try to make an uber-awesome costume your first time. I wish sooo bad that I had waited longer to do the Dark Kingdom generals, because I could have made them so much better if I had waited until I had a bit more skill. Lol.
Anyway. If you're really set on doing this, like avskull said, I think you need to start with an evening dress and just make really long, puffy tentacles as the skirt. Maybe find a pattern with godets and omit the godets, and just put some foam in the little "spikes" that you would have left over. I know Vogue has a great godet pattern out right now, (I'm using it for my Princess Mars costume ^_^) but I can't remember the number. Sorry! Oh, and just in case. Godets = little "triangles" that are inserted into the skirt of a dress to make it fuller.
Otherwise...@_@ you aren't going to find an Ursula pattern. I guess just use your imagination! Good luck ^_^;;
The next con I will be attending is [color=blue]Dragon*Con 2010[/blue]
Costumes for D*Con: Elphaba Act II from Wicked, Alice from Tim Burton's Alice, Lady Gaga stripe dress from Telephone.
Website:tavarieldesign.com ~ E-mail:email@example.com ~ I am an Undergrounder:ugcosplay.com