For writing a first impression, there really isn’t much there that is needed.
First off you should begin with perhaps a introduction, telling people what its about.
Then you talk about watching it and how it seems to you so far. This would include possibly information regarding story, characters, art and so forth. Just tell the world what you saw.
Conclude it somehow, either showing continued interest or dropping it.
That is the general breakdown of a FI. Happy reviewing!
Oh and tagging – Anime, Title, Character name, Genre, Season of release should be there, and others that you may deem necessary.