OK, so. Grab a world history study guide for a period you’re really into, and flowchart the salient events. Put each event on a notecard (let’s call “actual history” the pink timeline). Where possible, make some causal links.

Start messing with it! Try to ponder the “but for” outcomes, like if so-and-so died instead of lived, or if a piece of equipment failed at a critical moment (or didn’t!). Put those on differently-coloured notecards (let’s call those the blue events). Follow the chain of reasoning, as much as possible, until you have a pink-blue timeline, with all the events that you think would change and all the ones that stay the same, or close enough to the same. Keep it really general.

All of this is pretty generic stuff, so here’s where it gets hairy:

Create characters in the pink-blue timeline. Don’t tell your players what’s different from real history, but if they ask, let them know the scenario assuming those changes. Like, don’t say “this is the three-pronged Cold War if Hitler didn’t break the Nazi-Soviet Pact,” but allude to it: “it’s 1960, and you’re spies in the Third Reich capital of Berlin… etc etc.”

Once everyone gets started, and characters are introduced, indicate to the players that their characters remember history differently. They remember being spies in Berlin, but the enemy was the USSR, not Nazi Germany. Wait, didn’t the Nazis get wiped out…? What’s going on! Reveal the time travel angle slowly, and let them try to figure out the points of divergence. Once they have a time machine, let them go back to what they believe are the points of divergence, and change things. If they change a blue event and revert it to a pink one, great! If they change a pink event, well, then you need a new colour! Maybe green. If they change a blue event, but in such a way that the pink timeline isn’t re-asserted, you need yet another colour. They make orange notecards, right?

Play until the characters (or the players) are reasonably happy with their rainbow timeline.