The guarantee

Tell me what went wrong

Straight to my phone, and I will not make you argue for it.

On your confirmation. Not essential — I will find you.
Honestly, "it just was not for me" is a complete answer. The detail helps me not do it again, but it is not a condition of anything.

Within 14 days of the lesson or the review. Outside that, send it anyway or ring me — the window is there to stop me being asked about a lesson from last year, not to catch you out.