Open the game and click the workshop button at the bottom of the screen.

You can use this section to preview images and vote for skins, when hovering over a skin the voting option will appear.

To create a new skin, click + icon at the on top right corner of the screen.

Editing
You can then select the item you want to create a skin for on the left, this will open the workshop item editor.

From here you can now export and view the selected model and by clicking the texture square this will allow you to select a texture off your local system, you can also control lighting and enabled a backdrop for screenshots.

Clicking the small cross in the bottom right corner will reset it back to default or you can simply re-select the item and it will reset all values to default.

You can also control the color tint of the base color and other parameters using a color picker and value slider.

The Texture format is the following 
Base Color/Opacity - Color + Alpha Opacity Mask
Normal - Normal
ORM - Ambient Occlusion/Roughness/Metallic
Emissive Color - Emissive Color AKA Glow

Keep the texture power of two, ie, 256x256, 512x512, 1024x1024.

When baking textures in Substance painter the Unreal Engine 4 preset should be used to generate the correct layer format, Texture format should be always TGA.

We also don't allow exporting of current textures for various reasons.

Publishing
Once you're ready to publish, click the "Publish" button at the bottom of the window. 

When you publish your item it should automatically open steam displaying your uploaded item. If you're not logged in, make sure to, and then you edit and more screenshots if needed.

When switching to Publish Item the game will auto capture a preview image of your skinned item, you can also re-capture and adjust the brightness level using the camera button next to the preview image path.

We politely ask that you keep your Preview as the default generated thumbnail, if you choice to change this the image size can't be over 1MB and the image needs to clearly show the item.