It's simple, but it depends what you refer to when you say "guardian".Anyways.. if by "guardians" you mean the spirits around you or the spirits of your spiritual family you have to understand that, offering the EXPERIENCE of eating or drinking It's much appreciated than just placing food on a plate and leaving it on the altar. Another thing you can do - this one is more specific - if you want to offer food to spirits of nature (try to find foods which are more natural) You can place foods outside so that the wild animals can eat them. (I recommend fruits, It is very likely that those fruits will be eaten by birds which is also considered an offering)

Don't get too complicated just go with the flow.

instead of leaving a drink on the altar try to drink it in the Name of The Spirit and offer the experience to Them.

Spirits don't feel what us humans feel. We have access to this specific experiences of pleasure, which you can dedicate to The Spirits you Work with, or Worship.

NOTE:

-When I say Spirits I also refer to The Gods and Goddesses-

Hope it helps!