From the author's README on Thunderstore

Doom Retro (Old Gen)

The old-gen Doom Slayer from Tony Hawk's Pro Skater 3+4 as a full-body costume for skate.: the classic green Praetor armour with the grey visored helmet, exposed midriff, ammo belt, wrist blade, shoulder launcher, knee pads and brown combat boots.

Looking for the new-gen look? Grab Doom Modern, a separate costume.

Install

Install it from the Mods tab in the ReSkate launcher, then find Doom Retro under Apparel > Costumes in game.

Notes

Full-body costume: replaces your skater's head and body while equipped.
Built with ReSkate Studio on the Mousie costume layout.
Separate item from Doom Modern (new gen); install both and switch between them freely.
Works alongside the other Garetto THPS 3+4 costumes (Bam Margera, Chad Muska, Birdman).

Changelog

1.0.0

Initial release.
Full Doom Slayer kit ported: helmet, armour, arms, legs, wrist blade and shoulder launcher.
Seam fix across the back and waist, and cleaned-up boots.

Credits

Costume port: Garetto
Character model: Tony Hawk's Pro Skater 3+4
Doom Slayer is a character from DOOM by id Software

Install Doom Retro

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

Open ReSkateLauncher.exe and go to MODS.
Search for Doom Retro 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.