If you have a decently flat stone use it as your altar. Let's use Freyja the Norse goddess as an example. For her color it depends on her aspect. For the love goddess aspect golden and rose is advised. For the colors you can put a candle or two. Paint it the colors of your choice. That or you can just draw or paint her symbols on it. Say cats for example, paint one on there. For a food I would suggest strawberries. Sweet pastries are also good ideas. For a beverage, high quality wine or honey. Just normal honey would do well too. Don't let the food rot. As a love goddess, she likes it in mint condition.
Services offerings: You can offer sweet pastries in return for her help like these cases: a young soon to be mother. Ask her to help that person in the birth. Do it for a friend. That or a seidr worker. Rune basically is a seidr. Stuff to not let happen. As a love goddess, she loves offerings in aesthetic beauty. Don't let stuff rot. Hope this helps!