To use this Plex Metadata Provider, you need a RPDB API Key
Step 1
Go to RPDB Image Preferrences, choose your image styling preferrences and save your changes. (you only need to do this once)
Step 2
Choose metadata provider settings:
The API Key is not valid, please try again
Step 3
Let's install Movie metadata provider
Copy this URL:
Go to your Plex Server's Settings page, press "Metadata Agents" from the left side menu (under "Settings") then press "+ Add Provider" for "Metadata Providers"
Paste the URL you copied earlier, press "Save"
Scroll down on the same page and press "+ Add Agent" for "Metadata Agents"
For the title write "RPDB Movies", choose "RatingPosterDB Movie" as primary provider, choose "Plex Movie" as "Additional Provider" and press the "+" button to add it (optionally you can also add "Plex Local Media" as an "Additional Provider"), press "Save" to save your new Metadata Agent
On the Plex Settings page, press "Libraries" from the left side menu (under "Manage"), now press the "Edit Library" button (visible when hovering over a library) for the Movie libraries that you want to use RPDB images with
Press "Advanced" from the right side menu, choose the new Metadata Agent you configured ("RPDB Movies", if you do not see it in the options refresh the page) and press "Save Changes"
Step 4
Let's install TV Shows metadata provider too in the same way
First copy this URL:
Go to your Plex Server's Settings page, press "Metadata Agents" from the left side menu (under "Settings") then press "+ Add Provider" for "Metadata Providers"
Paste the URL you copied earlier, press "Save"
Scroll down on the same page and press "+ Add Agent" for "Metadata Agents"
For the title write "RPDB Series", choose "RatingPosterDB Show" as primary provider, choose "Plex Series" as "Additional Provider" and press the "+" button to add it (optionally you can also add "Plex Local Media" as an "Additional Provider"), press "Save" to save your new Metadata Agent
On the Plex Settings page, press "Libraries" from the left side menu (under "Manage"), now press the "Edit Library" button (visible when hovering over a library) for the TV Show libraries that you want to use RPDB images with
Press "Advanced" from the right side menu, choose the new Metadata Agent you configured ("RPDB Series", if you do not see it in the options refresh the page) and press "Save Changes"
Step 5
You're all set up! Now you just need to "Refresh Metadata" on your libraries to retrieve the RPDB images.
To refresh metadata, you can either:
Refresh it for an entire library, by going to "Libraries" (under "Manage") on the Plex settings page, then press the three dots icon (visible when hovering over a library) and then "Refresh All Metadata"
Refresh metadata for an individual item, by going to the movie / tv show page in Plex Web, pressing the three dots icon (to the right side of "Play" / "Resume" button) and then "Refresh Metadata"
Paste the URL you copied earlier, press "Save"
Scroll down on the same page and press "+ Add Agent" for "Metadata Agents"
For the title write "RPDB Movies", choose "RatingPosterDB Movie" as primary provider, choose "Plex Movie" as "Additional Provider" and press the "+" button to add it (optionally you can also add "Plex Local Media" as an "Additional Provider"), press "Save" to save your new Metadata Agent
On the Plex 
Press "Advanced" from the right side menu, choose the new Metadata Agent you configured ("RPDB Movies", if you do not see it in the options refresh the page) and press "Save Changes"
Scroll down on the same page and press "+ Add Agent" for "Metadata Agents"
On the Plex 
Press "Advanced" from the right side menu, choose the new Metadata Agent you configured ("RPDB Series", if you do not see it in the options refresh the page) and press "Save Changes"
Refresh metadata for an individual item, by going to the movie / tv show page in Plex Web, pressing the three dots icon (to the right side of "Play" / "Resume" button) and then "Refresh Metadata"
