Necromancy is a broad term: there’s psychopomp work, at one extreme, where you work harmoniously with various spirits to guide the restless dead (it rapidly goes beyond that, but that’s the root concept) - that’s taught in a lot of core shamanism trainings, in the UK & USA at least.
It’s pretty generic, I know a few people in my everyday life who’ve trained and do some work along those lines, and they’re not even “pros,” just into reiki and crystals and so on.
Then you have various types of ancestral worship, which is often a bit less altruistic, and then at the other extreme you have enslaving shades of various types, which I personally despise but it happens, and probably always has.
Oh, and there’s also mediumship, which can range from giving much-needed comfort & closure to the bereaved, to outright milking them, and even include charlatanry and “cold reading” - but it does lie within the broad remit of bothering those whose bucket hath been well and truly kicked!
The foundations of each of these are very different - for psychopomp workl you need a strong energy body, the ability to soul-travel, and to heal and channel energy; for ancestral work, it helps to have a broad framework, for example taken from the reconstructed Norse paradigms, Kemetic, ATR, or eastern ancestral veneration, mediums need good clairaudience & clairvoyancy (authentic ones do, anyway), and then for the enslaving thing you probably need a bunch of demonic help, but I know the least about that.
So, your foundation for each will vary, depending on goal and preference.