Jim expected to make Temporary Aprons to use when beginning and ending weaving. The slot in the cloth beam is so you can place the cloth with two sticks into it and wind up the cloth smoothly on the cloth beam. Read about a temporary apron in the Beam and Winding On section.
The beam slot in the warp beam is for the end stick and also so the warp can be wound smoothly on the beam. Read more in the section about Beams and Winding On.
When weaving the end of the warp, you’ll need to put in a temporary apron so you can weave up until the end stick is right behind the heddles.