I’ve tended to ask for a short meeting, tell them you’re leaving and hand them the letter at the same time.
They might treat the letter with nonchalant disdain but it’s better to get everything in order from the outset to save any problems should things turn bitter down the road.
There’s always a temptation to give them chapter and verse about their many and various failings etc, but it does you no good, if they were the sort of person / organisation to accept feedback from staff, they wouldn’t need it in the first place.
I think I’ve only ever had a single ‘exit interview’ they asked, I gave reasons in positive constructive ways, they told me I was wrong. If I’m pushed for a reason I say “money” it lets them know about the ruthless money focused mercenary I am ha ha.