We’ll start out with the pivot mesh.

Next we need to make a cage around our pivot mesh. Use the line tool to rough out a square around one end of the mesh. Be sure no portion of the pivot edges are outside the square.

Clean up your square.

Copy and paste your square to the other end of the pivot mesh.

Use the line tool to join your two squares.

This is what your cage should look like.

Next draw a crosshair inside your cage.

The bone upon which your restraint rotates should be placed at this crosshair.

Make an marker resembling an axle through the centre of the crosshair. The axle marker should extend beyond the ends of the lines making up the crosshair ...

... and beyond the ends of your cage.

We’ll see later on why we added an axle marker.