One of the easiest ways to work between the shoulder blades is to use a tennis ball. What you do is to lay on your back with the ball underneath you. Place the ball in such a way as to bring direct pressure to the problem area.
Breathe deeply then try to relax and allow the ball to sink into the knot. After a bit move the ball to the next area and repeat the process. Move your arm across your body to expose those hidden knots underneath and behind the shoulder blade.
The key here is to relax and breathe rather than trying to “grind” the knot out.
As with all of the tips at this site you should consult a physician as to the appropriateness of this technique for you.