Transformice Guide: Shaman Techniques:

Short Bridge

There are many cases where a small gap must be crossed. One of the most reliable ways of doing this is by using a global anchor large plank such that it cannot rotate. Well-placed large planks can cross almost 50% of the gaps present in the maps. However, if you misplace the plank and one of its sides is free to rotate, it will quickly turn into a deathtrap for your mice. Take your time and make sure you are placing it correctly; you only have to place a single object correctly to get it right.

Basic World Anchor

This is the traditional method of building a bridge. Select a plank, add a world anchor, and then ghost it. Place this flat against the floor or wall. You can now build your bridge using long planks and the immovable object anchor, simply by attaching them end-to-end.

Supports

When building a bridge, they have a tendency to bend under weight. To avoid this, use a ghosted short plank with a global anchor and let the bridge rest on it. Ideally, you should create the supports first and then create the plank above them.

Free Anchor Bridge

If you are using supports on a bridge, you can build the bridge using free anchors instead of immutable object anchors. The bridge will simply conform to the supports instead of being a rigid object. This is slightly more dangerous since it could easily end in disaster if the supports aren't placed well, but it makes it easier to move the planks around to avoid trapping mice. Overlapping World Anchors
By carefully overlapping two sets of world anchored long planks, it is possible to build a straight bridge. The problem is that the planks cannot be disturbed while the bridge is being built, or it simply turns into a death trap. As a result, this is often a noobish and rather poor technique for building bridges, and should be avoided.

Hanging Platform

A sometimes useful bridge can be made if the shaman is above the mice. A plank is given a world anchor and ghosted, then laid flat against a surface. Another plank is then ghosted with one end attached to the first plank, and then the final third plank must be attached by its center to the second hanging plank with an immutable anchor. The anchor attaching the second plank to the first plank can also be a free anchor, which will allow the platform to swing (which could be a good or a bad thing).