Okay, so I work with spirits, and in a Solitary way.
When setting up an altar, there are a couple things you should remember.
Find a quiet place that does not have a lot of people go by it
Be prepared to have offerings ready
Have an altar cloth
Have something the represents what/who you are setting the altar up for
Customise it to you and your spirit.
For example, I work with the dead. One of the ladies loved poodles. Adored them even, so I have a stone poodle for her.
Another thing you can do is find something that reflects their personality. If they are kind, loving, and nurturing, get them something like a dove. If they are courageous and loving, get a lion.
I like to set up my candles in a different way. I set them up so that one candle is in the middle, and the four others are in the corners. I usually set my offering, which is Brandy, in front of the middle candle, and off of the altar cloth.
When setting up your altar, it is important to let the spirits guide you as it really is for them, no? If you start to do something and feel something negative, that's spirit telling you to stop.
You may ask, "What kind of offerings should I leave?" well, it depends on who you are giving the offering to. The spirit may like ale, brandy, cookies, fudge, and almost anything else. When you give it to them, you should leave it for one to two days, and then you have it yourself.