Spirit guides are generally spirits that help guide you, hence the name. They may teach you lessons that are related to the situation you are going through. They will often show themselves in times where you need guidance or are troubled, however sometimes they will come forward if you meditate on them. You can also call out to them before you sleep, so there will be a chance they will manifest themselves within your dreams. However, I cannot garauntee that this will work, because they will only come forward if they want to.
Spirit guides can manifest themselves in a plethora of ways. They may take the shape of your spirit animal, a ball of light, or a regular person. Sometimes they may give inconspicous clues about being there by having that animal around you. Moths for example, you may notice them being everywhere around you. Or they may just be popping out at you for reasons you cannot explain. The animal they take form in may have to do with a lesson they may teach as well.
Many do not notice their spirit guides because they often linger. Or they may act as a regular spirit that is around you, like a companion. Everyone's experiences are different, and they will not all be the same.
My interpretation of Spirit Guides.