Now click on the retangle marque tool thing, go to edit>define pattern
Use: Pattern
Opacity (i suggest 20-60%)
Mode: Normal
(Note: If it does not show these settings you have a different version of photoshop ;))
Now go to your image, Edit> fill, Select your pattern in the square box thingy Opacity 20-60% Then on the layer you put it in(Should be new) Select the opicity to like 30-60% and your done!
Mine looked like this(Of coares its not a real sig, just colored green)