Watch this YouTube tutorial video, which should you guide you on the steps you need to follow to install it. Although it is for an older version of FM, the steps are still the same.  You need to use Winrar (or a similar program) to extract the downloaded file.

This is what the MP folder should look like for the respective version of FM you're using:

Anything beyond the blue line is up to you and how you want to structure your folders. 

The preferences should look like in the below screenshot:

To get them to look like this, navigate to your preferences on the welcome screen or by using the drop-down menu near the top right of the menu within your save game.

Choose the advanced tab (the blue square within the screenshot above) and select all from the drop-down menu on the right (the red square within the screenshot above).

Scroll down to the interface section near the middle of the list, where you are to make sure that:
• the “Use caching to decrease page loading times” box is unticked
• the “Reload skin when confirming changes in Preferences” box is ticked

After clicking confirm, it may take a few seconds or minutes for all the graphics to be loaded and you should now see all the graphics you have downloaded appear in your save game.

Once your skin is reloaded, you can re-tick the "Use Caching to Decrease Page Loading Time" button so that the splash screen at the start takes a lot less longer to load. However, the above process needs to be undertaken every time you add graphics to your game to ensure that the game "reads" them.

From FM20 onwards, there's been a substantial change to how Mac FM documents are stored. Previously the files would automatically go in the Documents folder here:

Users/YOUR Mac USERNAME/Documents/Sports Interactive/Football Manager 202x

Now the files will instead appear in the following location:

Users/YOUR Mac USERNAME/Library/Application Support/Sports Interactive/Football Manager 202x

By default this folder is hidden, so to access you need to do as follows:

Open Finder
Click ‘Go’ from the menubar
Press and hold down the ‘Option’ (Alt) key
This will bring up the ‘Library’, click to enter then go to ‘Application Support’ -> ‘Sports Interactive’ -> ‘Football Manager 202x -> graphics’ (create the last folder if necessary

Anything beyond this path line is up to you and how you want to structure your folders.

The preferences should look like in the below screenshot:

To get them to look like this, navigate to your preferences on the welcome screen or by using the drop-down menu near the top right of the menu within your save game.

Choose the advanced tab (the blue square within the screenshot above) and select all from the drop-down menu on the right (the red square within the screenshot above).

Scroll down to the interface section near the middle of the list, where you are to make sure that:
• the “Use caching to decrease page loading times” box is unticked
• the “Reload skin when confirming changes in Preferences” box is ticked

After clicking confirm, it may take a few seconds or minutes for all the graphics to be loaded and you should now see all the graphics you have downloaded appear in your save game.

Once your skin is reloaded, you can re-tick the "Use Caching to Decrease Page Loading Time" button so that the splash screen at the start takes a lot less longer to load. However, the above process needs to be undertaken every time you add graphics to your game to ensure that the game "reads" them.

You need to change your user data location. Simple steps on how to do so can be found here.

The above step does not apply if you’re playing on Gamepass, in which case the user data location has to be the default one (i.e. Documents / Sports Interactive / Football Manager 202x)

It is recommended that the user data location is not placed within the Steam installation folder as this may cause issues.

If you're using Gamepass, then you have to use the default user data folder - point 4 above does not and can not apply to you.