Use a support if needed, don't make it a balance exercise
Explore the movement, there's no need to hold a completely static position, unless that angle is the line you need to open.
Closed chain is usually best, not always, but usually. Plus, your bodyweight then helps pry the hips open.
Relax. Don't fight it, and don't rush it.
The mobility of the foot and ankle is as much part of the hip mobility as the hip is.
Work positions in both bent and straight leg, make sure you know the difference between straight and almost straight.
Strengthen the closing side after opening the opening side
Practice the skill that you want the mobility for after each set, let the body know what that mobility is for
Don't force, this shit can take time
Title Photo: WG-Fit client and Kyokushin Karate fighter Aneta Meskauskiene giving someone a bad day, by @Roman Odessky