1 The top section A is installed first. Then the two sides B and C. These go to the floor / threshold .
2 The threshold is typically a simple hardwood or stone strip. Maybe 3” wide.
Smooth and dense so the rubber engages well
3 Lastly the Automatic Door Bottom D is installed to the door slab, and fits in between Sides B and C.
Only cut the end without the metal plunger sticking out.
Discard the dust caps if it shipped with them, as they get in the way and drop performance.
Cut any of this with a power miter saw / chop saw. Use a standard woodworking blade, and can be carbide tipped if you like. The aluminum will cut like wood.
Always push the rubber into the aluminum channel a bit before making any cuts. The rubber will stick out a bit after the cut and act as a gasket that you can trim with scissors
You can apply a super thin bead of sealant to the backs of these components to frustrate a flanking path.