Lift the hands out to the side so that they are in line with your shoulders then bend the elbows to 90 degrees.
Place your arms in a door frame in this position so that your elbows and hands are both touching the door frame.
If you lean into the door you will feel a stretch in your chest (pec major). Adjust back slightly so that there is no longer a stretch.
Then press your hands and your elbows into the door frame without moving any part of your body. This is known as an isometric contraction as the muscles are contracting but there is no movement.
Press at a comfortable resistance (~20% resistance to start) and again make sure your arms do not move.
Relax your head and shoulders.
Hold for 10-20 seconds then release and repeat 2-4 times.
Repeat 2-4 times a day especially before activity and after periods of inactivity.
If you feel pins and needles whilst your arm is up at that level, lower a little so that the elbows are lower than the shoulder. This usually helps.