Here's a few hints to writing a good Althist (this is not definitive though):
Read a lot[]
Good timelines have lots and lots of research put into them. And not just deep, focused research: when you read widely, as much as you can that's related to the era you're working with, you always discover something unexpected that you can use in your setting. Althistories that don't have this research to support it tend to rely on popular misconceptions and stereotypes and aren't very interesting.
Points of Divergence (PoDs)[]
Chose your PoD carefully. Sometimes even the most insignificant change can make a great difference. If one PoD won't work, or the change is too unrealistic, use several PoDs to reinforce the change.
The Butterfly Effect[]
After the PoD, history diverges - moves in a different direction. Different timelines handle this in different ways. Some timelines dictate that everything must change following that starting point, either because the change itself affected the entire world, or simply because the author is philosophically committed to a totally different course of events. This means:
- People born after the PoD do not exist *there*, since their parents' lives were affected by the altered timeline and they did not get together in exactly the same time, place, and manner to produce the same offspring.
- What happens to people's lives is different. Financial situations change, perhaps with the roll of a dice or a toss of a coin.
- People die at different dates and/or of different circumstances following the PoD.
Other timelines allow for more convergence. Sometimes this is because the PoD only affects a single place or region at first, and the effects take time to spread out to the rest of the world. Other times, the author just decides that history will change less - one common choice is to have real-life people still get born after the PoD in order to populate the setting with familiar characters. This looser interpretation of the Butterfly Effect is considered a bit less realistic - "soft" althistory rather than "hard" - but can still make for a quality story.
Plausibility[]
It's up to you how realistic your althist will be. There are good timelines that are hard-nosed and concerned with the minutest historical details, and other good ones that are wild and crazy and filled with Alien Space Bats. But it's important to be intentional and consistent in your choice. And good research improves any timeline, no matter how realistic or otherwise its tone is.
TVTropes has an article on the Sliding Scale of Alternate History Plausibility that's a good overview of this concept.
Most timelines aim for some degree of realism. In these cases, always take time to think: "would that actually work, given the changes that have already happened?"
Characters[]
Don't hesitate to use obscure people. After all, following the PoD, they might have risen to prominence instead of others that did over *here* in OTL.
Don't hesitate to make some people up, should they be born after the PoD. After all, people born later might not even exist in the other timeline.
Additional advice[]
Above all else, don't forget to have fun.
If you need help, feel free to contact these guys.