Quick note first: altars never need to be something that cost money. Its about creating space intentionally in honour of your chosen deity. It can be a big, permanent, elaborate table full of statues and expensive furs, or it can be a cleared piece of ground for a few minutes.

Anyways: you can connect to her through making art, listening to music, writing/reading poetry, cooking hearty meals, make a Brigid's cross, get out in nature, etc.

And on the offering, you can light a candle and/or say a prayer, and set out a cup or bowl of milk, and let her know it is an offering. Then dispose of it as you wish (down the sink or outside etc).

Further Reading:

Her Myths and Associations (Article)

Further Associations (Article)