These work for me, I offer you these in hopes it can help you or set you on a Path of Self-Discovery to see what works for you.
1. Sit and just breathe. Concentrate on the in-breath, concentrate on the out-breath. Zone out background noise, only 'hear/feel' your breath. Long deep breaths.
VARIATION: breathe in for a count of 4 (or 3 or 2 if you can't do it for 4 count long deep breaths) - hold for a count of 4 - exhale for a count of 4 - hold for a count of 4 - repeat = inhale 4/hold 4/exhale 4/hold 4
2. IIRC, this is called 'progressive relaxation'.
Relax your body by tensing each body part up, and then let go/relax. Feel that letting go after the tension, that's relaxation physically, mentally just let all thoughts go. Proceed to next body part and repeat throughout your body.
When you need to relax, recall that relaxation after the body part tensing. If you cannot get into relaxed state during TT, physically tense your body and then let go. In time, you shouldn't have to tense up to relax, you will be able to go straight to relax.
If you need to 'anchor' it to some act or thing ... if you touch your racket a certain way, or bounce the ball off the racket, or bounce ball off the floor, or think of a word and associate it with the relaxed state, or a routine of a few motions and/or coupled with the 'trigger word'.
Personally, before a serve, i hunched my shoulders up to tense up, then exhale and unhunch my shoulders simulataneously, bounce the ball off the floor 2x and then serve. I can't get into a relaxed state right off the bat. Still need to physically tense up. Sometimes I just breathe in/out with 2 long breaths.
3. Close your eyes, visualize/recall pleasant times or a beautiful scene... mountains, fields, waterfall, virgin snow, good beer/drink, good food, sharing intimate moment with loved one, recall childhood vacation, etc. Feel how happy you are in that visualized/recalled scene. Feel relaxed.
At some point, you don't have to close your eyes, you should be able to get into that relaxed state.
Truly hope these helps.
EDIT: Baseball players have a whole routine, generally tar the bat, loosen/tighten their batting gloves, take a few swings then approach the batter's box ... once in, dig in and it's on
Basketball players before foul shots will bounce the ball a few times, look up at the basket, etc.
You get the idea.... good luck in finding what works for you!