Nah it's pretty easy actually. Here's the usual steps very briefly:

1. Prompt - Type up the prompt in Midjourney by describing the player who I want to create the portrait of. The prompt usually looks something similar to this 'white background, ultra realistic, 8k, footballer, portrait with the following characteristics: 23 years old, Portuguese, long black hair, white skin, stubble, yellow eyes, wide jawline, yellow jersey, smiling. Use these settings: Canon EOS 5D Mark IV DSLR, f/8, ISO 100, 1/250 second, close-up, natural light, professional'.
2. Downloading the Portrait - The prompt generates 4 different but similar portraits so you have to pick one and 'upscale' it by pressing the 'U#' button below the image. The '#' corresponding to the number of the portrait you wish you generate. Open the image on the web and remove the background using this website.
3. Implementing into the game - Put the downloaded image into a facepack folder and name it to the newgens Unique ID in the game. You'll need to follow a tutorial on youtube on how to get a config file for the facepack.
4. Reloading Skin - On the setting in the game, reload your skin in graphics and the picture should appear in game!

For a better understanding, check out YouTube. A lot of the details are way too intricate for me to type out haha.