“Appoint for yourself” is in singular (תתן לך), as if one can appoint judges all by himself. Obviously, that is not true, so why was this language used?
As Rav Chaim Vital explained, this is a hint. Everyone in the Nation of Israel has many Gates: Gates of Seeing, eyes; Gates of Hearing, ears; Gates of Speech, mouth; Gates of Smell, nose; Gates of Feeling, hands and feet.
Therefore, everyone needs to appoint judges and guardians at his own Gates: guard himself from seeing unseemly things; from hearing improper things; not to say foul words or bad things about others; not to smell the perfume from someone he should not; not to touch prohibited things, and not to go to wrong places.
What is the reward? - “Open the gates, and let the righteous enter!”
One also becomes deserving of the 310 worlds, reserved for every one who is righteous, and the gates of these worlds become open for him, since each world has a gate.
Art: Sebastien Stoskopff - Summer or the Five Senses