I believe that the best way to appreciate and comment on art is to first understand the artist’s intent. What was the artist trying to communicate or evoke with their work? Once you have a good understanding of their intentions, try to look at the artwork from that perspective.
Don’t be afraid to offer your own interpretation of the work, but always remember that it’s just your opinion. The artist’s interpretation is always more valid than your own. And finally, be respectful and constructive in your comments. No one likes a hater.