My mother is a daily user of the internet with extremely limited knowkedge and she has difficulty understanding even basic computer messages.
I designed a series of rules + practices which are stated as absolutes (i.e. no margin for interpretation) and they have worked well:
1. All emails with claims are false, even if I send them. Not only spam but also "snopes-like" scams from her friends. This rule always has precedence over anything else. 100% Never trust an email content. If it looks like there could be really bad consequences from ignoring an email, forward it to me and I'll decide.
I told her "imagine a stranger calls you on the phone and reads you the content of an email. Would you trust it?". She understood the metaphor.
2. She doesn't know her passwords. They are stored in the browser's keyring. Thus, she can't provide credentials to phishing websites.
3. She can click on links from emails, unless it is from a bank, because she knows her bank credentials. The combination of (2) and (3) makes the internet very usable for her as she can browse with confidence.
4. She only logs in to the bank website from her browser bookmark. She uses Safari's "Top sites" heavily, and she has learned to Google basic stuff.
5. If there is a weird message on a website, treat it as an email (i.e. it is false, etc)
6. Adblock is installed
7. She is beginning to recognize OS prompts, like icloud messages (storage, passwords). She knows she can never click on one before sending me a picture by IM. For these prompts password managers don't auto-input them and that's a problem. I must confirm its validity and then she has permission to open a notepad where her passwords are written and transcribe it to the prompt. But she always needs to send me a pic before opening the password notebook.
8. I have enabled Gatekeeper on the mac, thus she can't open binaries from the internet, only documents. Word macros are disabled.
9. If something doesn't look right, call/IM me on your cellphone
She is a very simple user and her needs are limited to websites, mail, office and a few games, so this works well. YMMV.
Maybe there are more rules that I can't remember right now, but the combination of not knowing her passwords + password manager + not trusting email + unable to run unknown binaries + adblock has worked wonderfully.
Let me know if you have more suggestions