I usually make offerings after spells. Whatever is symbolic to you, but you mentioned something about offerings. I would just dispose of the offerings, in a paper bag, after a day or so.
Well, during an offering, I generally feel some sort of presence and feel or see a sign. I always do my offerings outside, for various reasons, and I do my offerings in the same spot every time. Now it's like there is a small portal, because of the consistency. I mean an etheric astral like portal, one would have to be accustomed to, to even notice, but can generally feel. It is like if you draw enough symbols in the air, make your heartfelt offerings and say what you want to say, but without question when you say it, then you got yourself a spiritual portal from which the energy of wherever comes into your life. And I say that as my opinion, based off of my experiences.
When I give something, it is symbolic. I welcome whomever or whatever by drawing something in the air then saying something. I usually have incense burning, something that smells sweet, and I make my offerings in my favorite spots. Offerings don't need to be elaborate. All I say is that you gotta have that mindset that what you say, by what you feel, is truer than true without question, because if and when you start to question yourself in your path, the essence of what you want to believe has left you.
Have you ever quietly whistle into the wind, then listen and respectively wait for the wind to gust by? Then to have the wind calm back down, by giving it a lick of your saliva upon your finger? That is the same concept. An offering can be as simple as a lick if saliva on your finger. I am not kidding about the while whistling into the wind thing. Try it sometime, but remember to give the wind something in return when you call upon it.
Ok wait. Cast a spell to make an offering. I make offerings after I cast spells, with the idea that I am just worshipping my deities, and welcoming their guide and influence upon my life.
You know what would be an awesome thing to do is make a portrait of what you picture as your deities domain, tack it on the wall, charge and consecrate it, run it through a simple spell then you got yourself a spiritual window, where you can make offerings in front of. A place to meditate in front of. A place to do daily rituals in front of. So these portraits would serve as original, yet classic, portals at ones convenience. Not only that, they would be hidden in plain sight, but with a peculiar feel to it, but hidden. I think I just suggested an idea.