Offerings are common throughout many different practices. They can be a means of communing with entities, ensuring successful spellwork, giving thanks, honoring ancestors... They can be in the form of different foods, drinks, poems, prayers, money, blood, precious stones, carvings, ribbons, beads, water, or items of significance to that particular entity or ancestor. They can be as complex or as simple as you need or want them to be.
normally, you give an offering when in the circle, when you cast a circle, it become a sacred space where the God and Goddess are visiting with you and you are speaking to them, so they can see/hear you're offering something. another way to do it is to leave the offering outside and say a chant or blessing. myself i pass around cake and ale and the remaining is placed outside in my garden.
i'm in a coven of about 7 people. we have cake and ale [or wine and a cookie whatever available] each person takes a piece of the cake and a sip of ale but leaves some of both so after the ceremony i go outside and place it in my garden. i'm guessing you're solitary, so you do the same thing, take a bit, then leave it on your altar in from of your candles or statues representing the Lord and Lady. at the end of the circle, take them outside and place them somewhere in nature. [could be under a tree, in a pond, a garden, wherever]
I just tried an offering. I don't know if it worked or not.
if you felt the divine in your circle, or you felt something then you did it correctly. your first few times you might not feel anything [or it's so subtle you don't notice] simple things like you're happier, safe, or warmer, these subtle differences are how we tell if the circles cast correctly, or the Lord and Lady are presence. try meditating, think about the God then the Goddess and mark down if anything changed. could be a feeling, a sound, a smell, or a vision.
How do you know if the god or goddess heard you?
as i said above, you should feel them near you, if you do, then obviously they heard you. otherwise it could be something symbolic, say you place it outside and as you head back in you see a deer, or a family of squirrels come over to where you placed the offering. you could hear something, or feel something. it's different for everyone so it's hard to explain. i remember my first couple circles i was very nervous i did it wrong, i didn't really feel anything, i could hear them, or sense them. looking back though, i've always felt safe when i'm speaking with the Lord and Lady, or inside a circle.