I use CAMBODIA INTER TOUR & TRAVEL (one of 2 that seem the most reputable, the other being Maneerat). The Bangkok Bank process is pretty straightforward, IMO.
First, you’ll need to make sure your account transfer limits are at least 1,000,000. If you’ve not done this already you may need to visit a BB branch - they do a required face scan thing (fraud prevention) and any subsequent transfer over a certain limit (i think it is 50,000) requires a scan of your face via the BB app, to verify it is you making the large transfer. This has always worked flawlessly for me, no worries at all.
On the day of your extension, the agent takes you to BB - for Cambodia it is the branch close to Big C on Klang. There will likely be others getting an extension with you - the agent simply takes 800,000b and transfers it from one customer to the next - it gets transferred to your account, then immediately transferred out. You don’t need to give your account password to anyone, you really just need to know your BB PIN number (from the app) in order to transfer the 800k out of your account.
Works like a charm, if you ask me.