Hi,
Let me just add that of course in P1 there are no special high-symmetry points to guide you in the definition of the path, so you need to be a little creative. For instance, you can start from Gamma (0 0 0) and go to the edge of the FBZ along the b1 reciprocal lattice (1/2 0 0), to then go to (1/2 1/2 0), then to (0 1/2 0) then back to Gamma (0 0 0) and then to the edge along the b3 reciprocal lattice (0 0 1/2). Or something else! 🙂