Being objective is judging the work as the author intended it. Every author has a purpose and goal. To objectively review or rate one must identify those purposes and goals and make a judgement on how well they were conveyed across the story and whether they were achieved or not. Sometimes this may require context. Don't you ever wonder why critics go to extent of researching the author's background and life? They do so to get context which allows them to better understand the meaning behind the author's work as the author intended it. Say you read an instruction manual on how to build a computer but you instead wanted to build a light bulb, it objective when you say the computer manual is useless for building a lightbulb. However, the author never intended it for such so it should be obvious that it wouldn’t useful. This suggest that limits need to be in place for there to be a harmonious rating. These limit would be the intentions and goals of the author. Of course, they can have multiple goals so multiple responses might be necessary.

The second step is to determine how well it was done. This is essentially impossible unless you compare it to other works. I'll use an analogy to explain this. Say our goal was to hit a softball as far as possible, it can be easily determined how well you do based on how far you hit or whether you hit or not. However, how far is good? Is being able to hit 1m or 10m good? The only way to determine its value is to compare it to other players. Anime is the same. You compare works of the same genre or themes to plot it on the rating scale.

Also, how do we measure how well it was done? In the analogy above, you can measure the ball distance using a ruler giving objective information, but how is this achieved in anime? Well, we obtain this information by analysing the text. As I mentioned above, valid information is the information the author purposely sets out in order to convey his message and ideals. Using this information as evidence, we can support our claims regarding the anime.

Now, there's also bias which is the easiest to remove. Things like I hate all moe or shounens are bias. They need to be omitted to be considered objective. The full list of bias can be found here https://en.wikipedia.org/wiki/List_of_cognitive_biases.
One phenomenon that can be explained with the reasoning I used above is the 'so bad it's good shows'. They can be considered objectively bad because the show failed originally. The so bad it's good aspect wasn't the author's intention so it cannot be used to determine the value of the show.