If you pre-ordered, the game came bundled with some additional graphics, tilesets, faces, etc.

These can be found under your steam folder and then under steamapps\common\RPG Maker MV\dlc. It's safe to assume this is where future DLC will be added too.

• BaseResources contain some additional stuff for Sci-Fi.
  
• Cover Art Characters contain resources from the cover art. Don't know why that is a DLC, but hey, free extra resources :)
  
• Essentials Add-On contain some awesome tileset upgrades, along with faces and other resources. DONT FORGET TO GIVE CREDITS TO THE PEOPLE WHO MADE IT IN YOUR PROJECT! Credits are in the .txt in the respective folders.
  
• Kadokawaplugins and RPGmakerWeb_plugins both contain scripts for advanced users.

With the DLC, the resources usually come with sorted and in folders that correspond to where they belong in your project's folders. The resources inside steamapps\common\RPG Maker MV\dlc\BaseResource\img goes inside the "img" folder of your project. This is not necessarily the case with custom resources that you made yourself. I'll cover that a bit later.

So where do I put the resources?

The standard folder is Documents\Games\insert-your-project-name-here. Inside you will find, amongst other things, a folder called "img", one named "audio", as well as one named "js".

The "img" folder is where you will place all graphics from the DLC. Faces, battlers, etc., into their respective folders.

The "audio" folder is where you will place audio. Background music, background sound, etc., into their respective folders.

Inside the "js" folder there is another folder called "plugins". This is where you paste the java script plugins from the "KadokawaPlugins" and "RPGmakerWeb_plugins" DLC folders.

So, onto importing the DLC. The "BaseResource" dlc is simple, it's resources is packaged into an "img" folder and an "audio" folder. Just copy-paste these into your project folder and have them overwrite the existing "img" and "audio" folder. Don't worry, it won't remove anything, it will just add the new data, unless you have files that share their name, in which case I recommend you rename those.

The same goes for the "Cover Art Characters" DLC. The folders are not divided into "img" and "audio" but the bgm folder goes inside "audio", and the rest goes inside "img".

Essentials Add-On is the odd one out. Here some of the folder names are not the same as the folder structure used by the software. As such you have to manually copy and paste the contents of each folder where you want them in your project folder.

• The contents of both "BGM-MP3" and "BGM-OGG" goes inside the "audio/bgm" folder in your project folder.
  
• The contents of the "BONUS - Icons" folder goes inside your "img/system" folder in your project folder. Keep in mind! You will have to manually add the icons of this DLC inside the "IconSet.png" file inside the "system" folder of your project if you want to use them! This is done with any image editing software, I recommend GIMP as it is free and easy to use.
  
• The contents of the "BONUS - Titles" folder goes inside your "img/titles1" folder in your project folder.
  
• The contents of the "Facesets" folder goes inside your "img/faces" folder in your project folder.
  
• The contents of the "Tilesets" folder goes inside your "img/tilesets" folder in your project folder.

The java script plugins from the "KadokawaPlugins" and "RPGmakerWeb_plugins" DLC folders go inside the "js/plugins" folder. Due to lack of documentation, I cannot explain what each plugin does.

I won't cover required formats and resolutions here, just explain where you put your custom art. The formats and resolutions etc. are covered elsewhere.

To understand where to put your custom resources I will explain what to put in each fodler, starting with the "img" folder:

• animations: Here is where you put graphics that will be used as animations in battle.
  
• battlebacks1: Here is where you put the "floors" of the battle screen, where the battle takes place. They are used in the software along with the files from battlebacks2 to create scenery for your battles.
  
• battlebacks2: Here you put the backgrounds to the battle screen.They are used in the software along with the files from battlebacks1 to create scenery for your battles.
  
• characters: Here you put character's walking animations and other various animated sprites, such as chests, doors, switches etc.
  
• enemies: Here you put the graphics of enemies your player will encounter.
  
• faces: Here you put facial graphics of characters, used mainly in menues and dialoges.
  
• parallaxes: Here you put background graphics for the game world, such as sky backdrops etc.
  
• pictures: Here you add misc graphics that are used by the "Picture" commands.
  
• sv_actors: Here you put the side-view graphics of characters used in the side-view battle system.
  
• sv_enemies: Here you put the side-view graphics of enemies used in the side-view battle system.
  
• system: Here you put various icons used by the game's systems, such as game over screens, damage numbers, menu icons etc.
  
• tilesets: Here you put tilesets, the graphics that make up the game world
  
• titles1: Here you put main menu background graphics. They are used in the software along with the files from titles2.
  
• titles1: Here you put main menu foreground graphics. They are used in the software along with the files from titles2.

And on to the "audio" folder:

• bgm: Here you put .mfa and/or .ogg files that are used as background music.
  
• bgs: Here you put .mfa and/or .ogg files that are used as background sounds
  
• me: Here you put .mfa and/or .ogg files that are used as menu musics.
  
• se: Here you put .mfa and/or .ogg files that are used as sound effects.

Nope. Unlike the "Resource Manager" of VX Ace, the software search your project folder and add compatible files automatically. You are free to add the stuff in your game.

Thanks to a user comment, I found out a new place to put your resources! The same rules as mentioned before still applies, but instead of putting it inside your project you can put it inside your steamapps\common\RPG Maker MV\NewData folder!

That means that you can put all your resources in that folder instead of manually putting them inside each and every project's folder! That way all of your resources are always at the top of your fingers..! But it may also become cluttered as you add more and more stuff, so, keep things in moderation. Put stuff you only plan on using for one specific project in the project's folder instead :)

Thanks Trupsoft for sharing this!