Slide the galvanised steel Cross-Beams on to the supports and secure them through the pre-drilled holes and slots. The sliding nature of the Cross-Beams gives you adjustment in case your joists are unevenly-spaced apart. There is no need for the Cross-Beams to meet exactly in the middle.
If you're going to lay StoreFloor close to the edge of your loft space, then you may not have enough room to slide on the final Cross-Beam. In this case, just assemble the final set of Cross-Beams on to the Tri-Supports (without screwing anything in just yet) before placing the latter on to the joists, then lift them all up as one sub-assembly into place.