I do IF, 23:1, although it's occasionally closer to 20:4 if i feel like breaking with 2 smaller meals rather than my usual one meal a day. I was doing 16:8 before i knew IF was a thing and then just transitioned to one meal a day after i started keto, it feels the best for me.

Timing is going be be down to preference. Most people doing 16:8 would likely eat between noon and 8pm since it really just means skipping breakfast and then eating normally, but there's no reason you couldn't wake up and eat from 6am to 2pm and then fast, it's just likely going to be harder for most people.

As for what you can consume, there seems to generally be three camps.

1) Nothing except water, black coffee, tea 2) Nothing except the above and a tiny amount of milk or cream for your coffee/tea (<25-50cal) 3) Bulletproof coffee (up to 4-500 calories)

I fall into the first group - i drink coffee in the morning and water the rest of the day. I've always drank my coffee black so it was a non-issue. The argument for group 2 is that ingesting <50 calories doesn't break your fast and it makes the process more bearable for them.

The argument for group 3 is essentially that fat is metabolized differently than carbs and protein and you don't lose the benefits of fasting. I hate BPC, so i've never been interested in actually looking into these claims. Semantically, i wouldn't consider this option fasting.