Len.
You find the groove of the appropriate length for the handrail being formed. Bend a right-angle near the end of you handrail material, pass the short, bent bit through the inner hole of the selected jig groove, grip the wire to the jig, close to the edge and bend down into the cut-out in the edge of the jig, making sure the two bent down bits are in line with each other.
My experience with 4 mm handrails is that, because the etched holes in the jig are somewhat larger than the wire diameter, the waggle in the hole can lead to the finished handrail not being quite the expected length, so a bit of trial and error might be required. On the other hand, as your wire will be larger in 7 mm, that problem might not occur.
Dave.