make a new document with the size of 3pix by 3pix ( make sure itstransparent) then zoom in alll the way and use the pencil tool with it set to 1 pixel 100 hardness, then make 3 dots in a diagnal patern from top left to bottom right just 3 dots. kinna like this \ then go up to edit>define> define patern save it as what ever u wanna name it
then when u wanna use it make a new layer go to your patterns and click on it then use the paintbucket and click on ur image