I'll look for a website, but you can use a rag or a stippling brush.
To use a rag, you need to put a little paint in a roller pan. Tip it back until the paint runs back up on the flat part, let it drain a bit and roll the rag in it.
Very light, almost dry coats will look best because you can layer them and give it texture. It may look kind of bad when you start, but it will look better as you build it up. It may take a while, if you try to recoat it while it's tacky, the rag may stick. That could pull the previous coats off.
Take your time, you can do it. I've done it in school classrooms on block walls and drywall, It doesn't look like anything at first, keep at it, it looks better and you will be able to shape it up as you go.