If we use
as example
you'll go into the post, at the bottom you have to download the light or the dark version.
once downloaded, go into the download folder where you'll find the .rar packed file
Unpack the file then you'll have a folder called "mustermann23 dark 3.03"
inside that folder you'll see 4 folders
You can either copy all 4 folders into this location
Documents\Sports Interactive\Football Manager 2023\skins
Or just the 1 folder of which version you would like to use.
Then go into FM - preferences, search for skin and you'll see a box where you can pick the version of the Mustermann skin you've added to the Location above - reload skin and you should be golden
If you downloaded a skin with the .fmf extension, you do NOT need to unpack it, just copy the file to
Documents\Sports Interactive\Football Manager 2023\skins
Then go into FM - preferences, search for skin and you'll see a box where you can pick the skin you copy'd to the skins folder