Basically the same way: by finding and pressing 6 runes and then sticking a weapon in a given location. The one in the Mtn. Wilderness is the easiest of the three to open but the most difficult in terms of the monsters encountered. It does, however, have an excellent amulet as a reward. You need a long sword to open it and you just click on the various runes around the stone circle around it until they are all lit. They need to be touched in a particular order (unlike the others) but since you won't be able to successfully click them out of order, it's not a big deal.
The one in the N. Wilderness is arguably the most difficult to open because of the strung out locations of the runes, which are found on trees, while the location for the short staff is a rock. In difficulty it falls between the other two and the best weapon found in it is a Plague Axe...though there may be some +3 plate armor if I remember correctly.
For maps and other details on how to find the runes and rocks, I'd check out http://www.volberding.com/Wizardry8/Wiz8CemeteryWalk.htm and his map links from that page.