There's a couple of sub-genres - one where the change is just bizarre and unlikely, such as aliens landing in mediaeval Rome, and one where changes are more subtle and are more plausible outcomes to real events. Introducing magic into the world would probably put you in the first category, but you would probably want to know your period really well.

/r/worldbuilding would really be the best forum for help with this. I don't know what /r/AskHistorians' policy regarding alt-hist is but they would help with the details of the setting.

I would watch out, personally, for making assumptions about the past based on poor research. Once you get below the surface of just about any historical period, you see the nuances of the society you're studying rather than just the blunt bits. When I did legal history as part of my Masters, I was primed to write a paper being very critical of Victorian Poor Law jurisprudence, but ended up with the idea that they were all Scrooges thoroughly debunked. In most of the cases I looked at the magistrates and judiciary came up with equitable solutions which listened to the problems of the workhouse inmates just as much as they listened to the workhouse masters, and in one case the workhouse masters were on the side of the inmate, who had died through the neglect of a woman she left the workhouse to work for. So, even if you decide to put in a magical Pope or have Nostradamus be a real seer, or have demon-hunting during World War One, doing the research is still important. Even if you just learn that fish and chips was an invention of the era when fishermen could land their catch and have it shipped inland by the railways quickly and cheaply.

More importantly, read a lot of historical and alt-hist books - get a feel for the detail required and to what extent you can get away with broad-brush assumptions. My work was based on the kids' alt-hist series by Joan Aiken, whose work was outlandish and adventurous and envisaged a Britain still run by the Stuarts in the early 19th century, with the Hanoverians and then a breakaway northern English kingdom as the main antagonists (it had magic in it only in the latter books, written in the 2000s). It was decidedly cartoonish and all the better for it, but it was for children. I suspect to write adult alt-hist you will need to show your work, most importantly researching accurate social attitudes.