help - layer masks in photoshop

[Barracuda]

Hi this is griving me crazy.
Can someone tell me how to add a pattern i have created to a layer mask. I can create the layer mask but it is empy and i dont know how to turn it onto a pattern.Thanks in advance

 

[Barracuda]

thanks for the reply. I was trying to copy into the mask and getting frustrated when nothing happened. After much faffing about i realised that i can use the edit -> fill command and specify pattern in the command box. It works a treat.