From the author's README on Thunderstore

Manchester United 2007/08 away shirt for skate., made with ReSkate Studio on the relaxed tee.

Kits

Manchester United Away 07/08, RONALDO 7 (black with red piping, AIG, Premier League champions sleeve badges)

In game

Open Apparel and look under Tops.

Credits

All rights belong to the clubs, brands and kit designers. This is a fan-made mod.

Install Manchester United Retro Kit

You need ReSkate and your own copy of skate. on Steam.

Open ReSkateLauncher.exe and go to MODS.
Search for Manchester United Retro Kit and click install. The launcher also installs anything it depends on.
Press PLAY. In game, the MODS tab of the ReSkate menu (Insert) turns it on and off.

Or install it by hand: download the .zip and drop it on the launcher window, or unzip it into its own folder in Mods\ beside Skate.exe.

Maps show up in the ReSkate menu's level list. To join a server running a custom map, you need the same map installed. If the game won't load after installing, see known issues.

skatemods doesn't host or review this mod. The download comes straight from Thunderstore. Only install mods you trust, because scripts can run code.