From the author's README on Thunderstore

This model is from the fornite collab all rights and images are to their owners

Install

Install with the ReSkate launcher's mod manager (or drop the folder into Skate\Mods\), then pick Slender Man under Apparel > Costumes.

Updating from 1.0.0: let the mod manager replace the old version. Don't keep both folders installed: if both are present, the game uses the old one. Slenderman stays equipped; no need to re-select him.

Multiplayer

Other players see Slender Man only if they have this mod installed too.

Changelog

1.0.0: First release.
1.1.0: Fixed weight paint issues and grammar in read me.

Install Slender Man

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

Open ReSkateLauncher.exe and go to MODS.
Search for Slender Man 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.