This is my GO TO hip mobility exercise to mobilize the posterior/inferior capsule. If done properly, you should feel a DEEP stretching sensation in your butt. In this video, I am mobilizing my LEFT hip. You essentially want to shift your weight/hip over the hip you want to mobilize.
In the video on the left, I'll rest my foot on my other foot which puts my hip into external rotation. In the video on the right, I am in more of a neutral position. Play around with different angles of hip internal rotation and external rotation - so move you foot in different positions to mobilize the hip in different planes.
You should only feel a deep stretching sensation in the back of your hip, if you feel it in the front of your hip, you either need to change your foot position (most likely putting your hip in external rotation and moving foot closer/on top of your other foot) or you need to perform a banded hip mobilization. Exercise Library