Take a broom stick or something similar (not a barbell, it should be much lighter) and hold it in front of you. Push your shoulder blades as far forward as you can. Now, with straight arms move the stick over your head and all the way back down behind your back. ‘Trace’ with the shoulder blades, push them forward in the start, upwards as the stick reach the top of your head, and back as the stick is behind you. Move the stick back in front of you, again tracing the path with the shoulder blades. That’s one rep.
A lot of people cheat in this exercise by bending the arms and not paying attention to the shoulder blades - don’t! It’s not very useful if you skip these steps. Do it properly!
Picture 1. When the stick is in front of you, the upper back will appear rounded due to the shoulder blades being pulled forward.
Picture 2. When the stick is above your head ,the shoulders will be shrugged upwards.
Picture 3. When the stick is behind you, your chest will be pushed out.
If you can’t keep your arms straight you’ll need to widen your grip. Keep widening it until you can move full range of motion with straight arms. Over time you’ll be able to narrow your grip.
You can make this exercise even better if you put a small weight on the middle of the stick - a 2.5 kg (5 lbs) plate is enough.
If you don’t have access to a stick or broom you can use a rubber band. If that’s not available you can simply do arm circles explained below. They’re however far less efficient.