Lie on your back on a firm surface.
Relax your arms by your sides.
Draw your shoulder blades back towards your spine, flattening them against the surface.
Whilst keeping your shoulder blades still, lift your weaker arm off of the surface and place your palm on your opposite hip
Take your arm from your hip, diagonally up out above your head, in a movement similar to drawing a sword.
Remember to keep your shoulder blade flattened against the surface throughout.