Disclaimer: I’d like to make sure it’s understood that these are in no way to be taken as qualified instruction. I suppose that technically I should classify these as ‘reminders’, but it is possible to learn from them… Just make sure you check with someone that actually knows what they’re doing before you try and do them halfway up a cliff.
Literally the most simple knot you can tie. Not much else to say.
Take the tail and pass it over the rope.
Pass the tail up through the bottom of the loop.
The same can also be done with a bight, if you want to tie an overhand in the middle of a rope, for whatever reason.