A physical alter would have an object of your choice. Something in which you had unshakeable Faith, let's say a picture of the Christ or Buddha, etc. You can also make a circle or use the image of a lighted candle to enhance your concentration.
Alters generally have burning incenses for fragrance and atmosphere and to attract inner or psychic beings. They tend to have flowers which are fresh, pure and also fragrant and useful for atmosphere. The basics for an alter would be a cloth on a table or chair, preferably white as a symbol of purity. The other things would be on top.
My shrine at home (alter), is purpose made. I have a picture of my Spiritual Director in the middle. I also use incense, candles and flowers.
This is the physical alter, Andrew. The inner alter/shrine is inside you. The body is the Temple and the Soul is the Shrine. In the preparation of the inner shrine, purity, discipline, love, compassion and selfless service are essential. If your question is about the inner alter, then write to me. Otherwise the things that I have mentioned for the outer alter, is more than sufficient.