[Retroman]

There should be plenty of tutorials on google =] google is everyone's friend.

You most likely need to create/download brushes and learn how to merge them with your image if you're wanting to blend properly.

Tweaking the opacity settings is usually a must, but won't look good on its own.

Besides that, it's all about being creative and having a keen eye for detail, and what works well. Ive only just got photoshop CS2 back on my computer this week, so im a bit rusty, might be able to help better later...if this hasn't been answered by then.