Power of Attorney asap, if you don’t already have it.
All the following are from my own experience of 2 close relatives, but I’m not saying they’ll apply to every case.
Don’t try to correct them if/when they come out with weird stuff. If they had tea with the Queen last week, just say, ‘Oh, how lovely - what sort of cakes?’
Ditto when they have an angry bee in their bonnet about something that’s all in their own head, E.g. if they insist that neighbours/whoever have stolen something, say ‘Dear me, that’s terrible, I’ll get on to the police first thing tomorrow.’
(You can probably can repeat the same things ad lib, they won’t remember that you said it X times before.).
Don’t bother trying to write things down to remind them of anything, ditto e.g. ringing to remind them to take medication. They will forget as soon as they’ve turned their eyes away, or have put the phone down.
If they forget that someone’s died and ask where they are, and get very upset when you remind them, the next time (if they ask again soon) just say they’ve gone to the shops or to visit Auntie So and So.
Make copies of keys! Hiding such things away so successfully that you will never find them, is often a feature of dementia.
By a certain point, it’s likely that 24/7 care and supervision will be needed, which will usually mean a care home. Pious, sanctimonious people, who have very likely never had to live with dementia, will tell you how dreadfully selfish and uncaring it is to put a parent in a care home.
Ignore such people.
I’d just add that despite all the horror stories, very good care homes do exist, and they’re not necessarily by any means the most expensive. But do be prepared to look at many - and write off any that ask you to make an appointment for an initial visit.,