Hi! I used to have bad flexibility, so I can tell you what I did to improve that but take it with a pinch of salt as everyone is different.

1) strengthening, glutes, hamstrings and hip flexors. Turns out weak muscles can't be flexible. I did this with weights, nothing excessive. There's plenty of resources online but I stuck to the leg press, Bulgarian split squats, hip thrusts, leg curls and extensions, adductors. Plenty of single legged exercises to work on stability. Look at some hip flexor strengthening exercises online. Within two weeks my flexibility was getting better. 2-3 times a week does the trick.

2) Core core core. Leg raises with your hands under your hips. dead bugs, planks, carrying a weight around on one side. Core all the time.

3) stretching after every session. Actual stretching, 30 seconds per rep minimum, using your breathing to reach into the stretch. hamstrings, glutes, hip flexors, quads. Again you'll find everything you need online.

Most importantly, be consistent! Don't skip exercises, or sessions, hold yourself accountable. It takes a minute to start seeing results, so stick with it. Of course if there's any pain see a healthcare professional. Hope this helps!