“The border turned westward to Aznoth-tabor.” It seems clear that Joshua is doing what he has done for other tribal measurements: picked a city as a starting place and gone out from there. The city is likely Heleph (Josh. 19:33), and from there he went east to the Jordan, and now he is describing the border in a westerly direction. It seems clear that since the first city is Aznoth-tabor (the “ears of Tabor”), that Heleph is near Mount Tabor, likely slightly east of Mount Tabor because he went west to Aznoth-tabor.
“as far as Zebulun.” That is, as far as the tribal territory of Zebulun.
“and to Judah at the Jordan.” This is not the tribal area of Judah, but a town called “Judah” on the Upper Jordan (north of the Sea of Galilee). The location is unknown.