I'm not fond of the term "spirit guides" because it implies some special status that every magician needs. Depending on how you define the term "guide," every spirit can guide you in some way. Here, I define a spirit guide as a soul who can lead you to answers, messages, or pathways you were previously unable to reach. Although some people assume that they can only have one spirit guide, rest assured that most spirit workers contact multiple. Here's how spirit guides help you in death witchcraft.
Although spirit guides are often over-emphasized in generic spirit work books, they're frequently overlooked in necromancy. In death magic, spirit guides can lead you to the afterlife, connect you to other souls, and deliver messages from other entities. They may aid you in certain divination techniques such as scrying, or they may appear in your dreams to point you in the right direction.
Finding your spirit guides takes time. Although many authors write rituals to summon your spirit guide, I personally don't like these. I think that calling out "who's my spirit guide?" to the void is a perfect opportunity for spirits to take advantage of people, especially new practitioners. Don't force it. Guides always come as you expand your craft.
If you are interested in death witchcraft or necromancy, start how most of us did: by contacting souls through divination, graveyard dirt, or bones. Over time, you'll find a soul who is willing (often eager) to show you secrets and paths you never dreamed of. You don't require a spirit guide to practice; you just need yourself.
How do you know if a spirit is your guide? Figure it out yourself. Is the entity reliable? Have they answered your questions correctly? (Asking them something you already know is a good way to verify divination.) Do you have good reason to trust them? Have you worked with them before? Often, a spirit becomes your guide before you even think to apply the label. As with all things spirit work, let this happen organically. Your death will flourish from it.