How to proceed with a compensation claim against an airline

I was recently going on holiday and my arrival at destination was delayed by 24 hours.

My journey involved two flights, both booked at the same time directly with the same airline. The first flight was delayed which caused me to miss my connection and the second flight only runs once per day so I had to wait for the next one.

The airline was very helpful and provided me a hotel and meals for my extended layover.

However, on doing some further reading on Citizens Advice it looks like I am also due some compensation.

My journey origin is in the EU (Scotland) and my final destination is outside of the EU. The distance is more than 3500Km so it seems pretty clear cut that I should get the maximum payout, except from one passage that struck me as an easy get-out for the airline

You’re unlikely to get compensation if the delay was because of something outside the airline’s control, like bad weather or a security risk.

I asked at the check-in desk what the reason for the delay was and they were very vague. So how can I know if it was the airline’s fault or not?

I have searched the airline's website (Qatar) and they state exactly the same information as citizen's advice but don't seem to offer any easy way to claim. It seems like it should be quite a transactional thing with no room for interpretation so it feels strange to me that there is no standardised way of proceeding.

When searching online for further advice I am drowned in companies offering to make the claim on my behalf, but I expect they will come with a high fee so I'd prefer to just do it myself.

Is it really just a case of me writing them an email or have I missed something important?