Players who have authenticated can use the /skin and /se commands. When you use /skin a UI will open up. Click on any skinnable item, such as a shirt or an AK. All available skins will show up. Find the one you want and click on it. Your item will then be reskinned.
If you have deployed an item, such as a door or a box, you can use the /se command to skin the item without picking it up. The UI is the same as above.
If you know of skins you’d like to have on the server, join Discord and submit a support ticket with a link to the Workshop item you’d like added.