This is the DIY way of relaxing, I do it when my worries stop me from going to sleep:

Lie down flat in bed, curtains closed, make yourself comfortable. Close your eyes.

Now start on your feet. Visualise your feet, and relax them completely, toe by toe, concentrate on the feel of them as they relax. Then the top and underside of your feet. Once you're used to doing it, you'll feel them feeling heavier.
Then do the same with your ankles, feel how tense they are and relax them.
Then your calves.
Then your knees.
Then your thighs.

After that, move on to your fingers. Relax them completely, take your time. Once they are completely relaxed, move on to your wrists, your arms, elbows, upper arms.

Back down to your pelvis, your belly, your diaphragm.
Your spine, start from the coccyx and relax vertebrae by vertebrae upwards up the neck. Now concentrate on your shoulders and neck.

Finally, your face. Relax the muscles in your mouth, your eyes, forehead, even your ears.

Don't rush. Take as long as it takes to relax all of those muscles, it gets easier with practice. I am usually asleep long before I get to the face.

By concentrating on this, you are emptying your mind of everything else, it is probably a form of self-hypnosis, but it does work wonderfully.

I also do it for my kids at the dentist to take their mind off the injection, make them concentrate on their breathing, on paying attention to how they breathe, in through the nose, out through the mouth, feel how your stomach inflates and deflates, that kind of thing, very effective. :-)