With the spacer block installed, re-position the miter gauge into the left slot of the table saw. Then, remove the two screws that mounted the jig to the miter gauge.
Slide the box joint jig a bit to the right, using the remaining portion of the spacer block that you cut off in the previous step to adjust the width of the jig. See the image above for clarification.
With the jig positioned so that the installed spacer block is now precisely one box joint finger's width past the dado blade, re-attach the backer block to the miter gauge using the two screws you just removed.
The box joint jig is now complete. In the coming steps, you'll learn how to use this jig to cut perfect box joint.