Kelly,

It is a Tee with a top branch made of clear tubing.  The side of the Tee is connected to the spark plug location.  Using some fuel line (anything handy will do), I connected this side branch to an old spark plug with the internals removed. An o-ring seal was used on the spark plug instead of the metal one so hand tightening was enough.  The bottom of the Tee is connected to a balloon with soapy water in it.  Squeezing the balloon and releasing it will leave a soap film across the side branch.  Turning the eccentric shaft will displace the soap film up or down the top branch of the Tee.  It will take a little trial and error to get close to TDC where the soap film isn’t blown completely out of the top branch.

Steve