Ditto to what Jeng said about keeping shoulders back and down. I did the exact same thing practicing my chopper (same move Jeng said, different name???) and Veena met me in chat one day to show me the right way to do it, as well as some strengthening exercises that target your shoulder blades and the back in that area.
For the strength training, look up Veena’s video on shoulder strengthening exercises (it’s on YouTube and here). She has a thing called ITYs or TIYs or something like that. (the letters refer to the position of your arms) But watch that video, then do what she says for the T position, except hold your arms further back and it’ll target more your shoulder blades and such than what the exercise is originally meant for. That’s what Veena told me to do.
Hope this helps!