Lots of stretching and mobility practice. First- make sure your muscles are very warmed up before you start stretching for flexibility training. Look into foam rolling. It will aid in loosening and prepare you for a deeper stretch.
If you subscribe to Veena's lessons- she has a lot of exercises and a long video on improving back mobility which incluldes a decent amount of foam rolling. https://www.studioveena.com/lessons/view/4f5fbd45-6c18-4371-84a6-3f4a0ac37250 If not, you can find decent video demos on youtube. Cat/Cow, camel, cobra/half cobra, and bridge are my favorites. King pigeon is also good but very hard if you're very tight. If you can use yoga straps while practicing you can work your way to it slowly and safely.
Also, make sure you're doing strength conditioning at the same time. The back is very susceptible to injury if you increase flexibility rapidly without corresponding strength training. Again, Veena has some good videos on it in her conditioning section. I feel like I get the most out of supermans.