Harness the horses - namely, to the war chariots, for which Egypt was famed (Exo 14:7; Exo 15:4).
get up, ye horsemen - get up into the chariots. Maurer, because of the parallel “horses,” translates, “Mount the steeds.” But it is rather describing the successive steps in equipping the war chariots; first harness the horses to them, then let the horsemen mount them.