Describe. Give a distilled overview of what you saw from start to finish. It doesn't need to be detailed; it just needs to convey the overall impression and trajectory of the performance. Mention the title of the ballet, the choreographer and the name of the company who performed it and when.
Analyze what the ballet communicated to you. It may have told a story or it may have been an expression of a mood, an emotion, an idea. Decide what it was trying to say and describe how, why and how well it was said.
Write down the narrative arc of the piece no matter how abstract. Most works of art have a beginning, a middle and an end. This does not have to be literal or technical -- you can also describe the emotional journey the ballet took you on.
Evaluate elements of the performance wherever in the critique it makes the most sense to do so. Give kudos to the performers who stand out and make note of the theatrical elements (music, lighting, set and costume design) when they are exceptionally good or bad. Speculate on why things worked out or didn't. This is the place for constructive criticism.
Summarize your opinions about the ballet. Try always to end on a positive note, especially if you have been very critical. Ballet is hugely difficult to choreograph and perform; your critique should indicate an understanding and appreciation of this.