The first part of this verse records a flashback. At the end of Judges 3:15, we read that the Israelites sent their tribute by the hand of Ehud to Eglon Judges 3:17a then says that Ehud brought the tribute to the king. Judges 3:16a is then a flashback to a time when Ehud made the sword, seemingly before Israel actually sent him to Elgon.
16 And Ehud made for himself a sword with two edges, a cubit in length, and he bound it on his right thigh under his clothes.