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Every second a full second passes

I've specifically avoided time travel in most of my stories because it's a pain in the ass to make work.

So, I'd go with the most forgiving version where the act of time travel splits off a new timeline. Or, that the travel itself is to another timeline, depending on how you want to look at it.

That's the easiest to write around, imo. It's also the version most likely to hold up to scrutiny, even if (when) I made mistakes. You can hand wave a lot of stuff when you don't have to worry about paradox.

I don't understand science, so my character would have to get some kind of time machine that they didn't understand. But was really simple to use.

The "uncaused action" version, similar to Stross's Palimpsest. The traveller goes back and by changing the past creates a future where the chain of events that led them to go back may no longer occurr.

With a dash of "anchored time device", as seen in Primer, where the traveler can't go back any further than the activation of the device that brings them back.

Avengers Endgame had the most logical form of time travel prior to Captain America returning.

It depends on the story though. "Illogical" time travel is still fun and it may better suit a story that isn't meant to be rigid science fiction. "Back to the future" style photographs changing while you look at them is great for more light hearted stuff.

I also really liked Tenet. The reverse entropy weapons made no sense but the reverse time people made more sense and it was cool.

It just works. Turns out all the time paradox stuff is nonsense and anything is possible.