Weather to leave her or cut it open is up to you. We rarely cut them open, unless they are really big or she is going downhill (although I don't really think it helps anyway). If you leave it there is a possibility that it will get bigger and leave a large lump of scar tissue. I would likely leave her alone until after she calved unless it was causing a problem.
Now to cut it open, I would restrain her exactly as you say. Feel around on the lump for a soft spot, and if there isn't one, you may as well just leave it since you won't get much out of it anyways. If there is a soft spot, take a scalpel, and make a decent sized incision into that spot (which hopefully is on the bottom side). Then squeeze all the gunk out, and useing a syringe (without a needle) flush it out with diluted iodine. I wouldn't worry about anasthetic, and this shouldn't take too long.