Ugh, this is so difficult to answer because it’s almost something that I physically have to demonstrate on a person.
I personally have TMJD. Which, sidenote: it is not TMJ like most people think, it’s TMJD. TMJ stands for the temporomandibular joint, aka the place where your temporal bone (side of your head) meets your mandible (lower jaw); everyone has two TMJ. TMJD stands for temporomandibular joint disorder, which can develop due to anxiety, poor orthodontia, and other factors. It can cause jaw pain, headaches, tinnitus, and lockjaw.
The thing that has most helped me is intraoral self-massage, which is hella difficult to describe over the internet. Um, you can watch this video? You can’t really see what’s happening inside someone’s mouth, though. Basically you are hooking your finger in your cheek, sliding it to the top of the cheek, and pulling it up and outward at a 45 degree angle. Hold there until you feel a release. That’s hooking into the masseter muscle and stretching it out.
I hope that’s helpful in any way. If you’re ever in Portland, Oregon I’d love to demonstrate it on you in person.