Players who have authenticated can use the /skinbox and /sd commands. When you use /skinbox a UI will open up. Drag any skinnable item, such as a shirt or an AK, to the boxes on the right side of the screen. All available skins will show up. Find the one you want and drag it to the left into your inventory. Your item will then be reskinned.
If you have deployed an item, such as a door or a box, you can use the /sd 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.