First of all, I'm going to assume that you mean Aesthetics.
You need to first identify what type of mood your gametype creates. For an Infection map, you might want to create a tone of fear. A Big Team Battle map would emphasize the theme of massive armies clashing. If you are doing a themed game mode, you would want to have specialized aesthetics (ex
Jurassic Park map).
You also need to identify level of detail. If you are making a slayer map, maps are often pretty simple or plain aesthetically so that the visuals don't interfere with the gameplay. For something like Infection, the aesthetics are much more detailed to create a level of immersion.
After you've identified the mood, you need to find aesthetics map elements that match it. There's no right answer to this one, but looking at some maps that set a mood similar to the one you are looking to achieve can help.
Good luck with your Forging career, and hopefully I'll see your map someday.
EDIT: TIL that esthetics is an alternate spelling of aesthetics (thanks Wikipedia).