Before I get a bunch of people yelling at me that witches aren't real and whatnot, I am not talking about the broom riding crooked nosed witches that actually have powers. I am talking about people who believed that they could conjure spirits and whatnot for magical purposes. I figure these people had to have actually existed at some point because the myth would have had to have started somewhere and there are people today who attempt magic or believe they have magic powers.
My main question is about the sorts of rituals and such that one of these people would have performed, if people would have known that they were doing these things, and what their intended effects were. I am thinking about writing a story with magic in it and I like to base my fiction on stuff people actually believed was true. I haven't been able to find much about this on my own since every time I look I just find hippie stuff about how to perform magic, or dubious sources at best.
Keith Thomas, Religion and the Decline of Magic, is a tour de force. It is late medieval/early Renaissance, but it will give you great insight into what people believed about magic and witchcraft in England.
Medieval witches probably practiced Christianity, like most people in medieval Europe.
There's been a tendency in recent movies to portray witches as being actual devil worshipers (for example, in Black Death [2010]), who were being persecuted because they were following religious traditions that actively threatened the Christian establishment. There's some academic support for this idea - Carlo Ginzburg, for example, has argued that paganism survived, secretly, in medieval Europe for 1000 years, and that this is the witchcraft that church leaders were so concerned with rooting out in the late medieval / early modern period. But this argument is really stretching the available evidence. And, I'll argue, dangerously confuses rhetorical tropes with reality.
The idea that witchcraft is an ancient, anti-establishment tradition that has been standing up to Christianity (and suffering persecution) for thousands of years is a really attractive narrative for many people in the present who want to connect their modern magical practices to an imagined something in the past (as you've noticed), so the idea of an ancient tradition of witchcraft is hard to debunk. And because it makes good film, Hollywood has jumped on the bandwagon, and treats witchcraft as a real religion, rather than the horrible side effect of early modern authoritarianism that it was.
Most of what we know about witchcraft comes from the people who were identifying and persecuting so-called witches. There's little to suggest that the women they targeted were actual devil worshipers: they seem to have usually been women whose social positions (often, widows) left them on the margins of society. Much like today, medieval and early modern authorities maintained and strengthened their power by attacking marginal groups (frequently the Jews, but also these so-called witches). But there's little reason to believe the charges they made against accused witches were based on real things the women had done. Much like antisemitism, the accusations were coming out of Christian beliefs about what witches should look like, not responding to the actual actions of the victims within their communities. Believing these accusations and treating them like they represent real beliefs is dangerous, and is rather like telling a story in which a greedy medieval Jew drinks the blood of Christian children (that is, treating a hideous stereotype like a social reality).
There were, of course, lots of kinds of magic in Europe, treated with varying degrees of suspicion by the establishment. A lot of 'magic' was assimilated into the church - for example, in the early middle ages, pictures of Jesus and the apostles were added to pre-Christian (Roman) medical texts to make it clear that their contents were ok (see this forthcoming dissertation). Monks in 10th century England copied a book of magic spells that used bible verses, herbs, and other treatments that give every appearance of coming out of pre-Christian practices to heal ailments. Christianity was usually flexible enough to absorb this kind of healing and herb lore. The story you get in the TV miniseries World Without End (2012), where the woman who cures people with herbs gets hanged for her troubles, is more stereotype than reality - those kinds of magical healing activities were widespread in Christian Europe, and didn't usually mark someone out as a witch (even if they had a lot of non-Christian content blended into the things they were doing). Medieval Christianity had a lot of room for diversity, except at a few flash points when local authorities wanted to solidify their power by finding a common religious enemy.
To read more about the history of accusations of witchcraft (including the kinds of things witches were accused of doing), see N. Cohn, Europe's Inner Demons: The Demonization of Christians in Medieval Christendom (Uni Chicago Press: 1993).
For Ginzburg's attempt to reconstruct magical practices in pre-modern Italy (which are a great example of how much magic could fit into the lives of Christian peasants): The Night Battles.
For Ginzburg's (problematic) argument that witchcraft was a real religion: Ecstasies.