There are so many different approaches. Was going to direct you to SBB tutorial section but the only one I could find was for Paint Shop Pro. You could try searching the tutorials at other scrap sites. Here is a thread I found by doing a search in this forum.
Torn edges in PSE
I often use a combination of things I have learned from different torn edge tutorials. I need to get off the computer for now but will try to come back later and give you a rough sketch guideline sometime tomorrow.
Also SBB user Carinspixels had a bunch of torn edges on her blog awhile ago. Here is the link to that post in the blog freebies notifications forums
http://scrapbook-bytes.com/chat/showthread.php?t=28210
Don't know if they are still available.
Once you get your torn edge made you can save it as a PNG file and reuse as a template for future LO! Have fun learning.