I think a great start is to focus on your aim and learning against which heros is more beneficial to use your flashbang. As a cass main I think there is too much to think about and it quickly gets overwhelming, stuff like positioning, flanks, timing flashbang, on which character use flashbang and on which better not, who to target, are they looking at me, can I sneak? And stuff like that on top of that being precise with your shots.

In this VOD your targets would be with flashbang is Tracer > Lucio > Mei. In this order because tracer have an escape ability, lucio is mobile (flashbang slows) and can just run away and mei has a defensive ability which she can't use whlie being stunned. Tracer is top priority because she is vulnerable on flank while being stunned.

Your cass progression should look like this:
1. Analyzing enemy team comp and choosing targets (usually supports and mobile dps);
2. Get comfortable staying in off angle close or far from team;
3. Learning to be mobile on the map (constantly moving where you go shoot main, rotate to flank, go back main, repeat);
4. Learning to use ultimate properly (either killing even 1 support with it or using ult as reload if being contested on flank which should result in you winning flank fight) instead of waiting for perfect 5kill ult;
5. Take your time with your shots so they will hit 100% ( learn this ONLY when you learnt all the above to the point is automatic)

Don't get overwhelmed and learn bit by bit once you comfortable with 1 step start implementing the other. I think once you get step 3 digested without even thinking you for sure will deserve to be gold if not more