1. First make a document at 500x500 pixels and 300 ppi
2. Fill the document with a neutral grey. You can do this by using this keystroke [SHIFT + COMMAND/CONTROL + 4] and choosing 50% grey. You can also make whatever grey you want using the Paint Bucket tool.
3. After filling the canvas, use a Stained Glass filter to make the initial texture using the setting I have listed.
4. Finally copy the current layer, set the copy to 50% opacity, and use the Move Tool [V] and arrow keys to move the copy up every so slightly.
5. You're done! now just export as pattern and the chainmail will appear in menu when ever you wan to insert a pattern.