the basics - scoring with a bone folder
The bone folder is a great tool to give a project a clean look. It is used for scoring—which is the act of making a depression into paper so it can fold more easily. If paper isn't scored before creasing, it can crack and the fold will have a ragged edge.
I like placing the bone folder on the point where I need to score first. Then I slide the ruler up against the bone folder. By doing this I allow for the thickness of the bone folder. I know this is minimal and almost negligible but in some instances (like making boxes) that can make the difference between a crooked or slanted box and a straight one.
Drag the bone folder against the ruler until you've reached the end point of the desired fold and you're done. Do not try to re-score any given line. It is virtually impossible to score in the same place twice and your paper will show two marks instead of one clean one. Just make sure to press hard enough the first time.
These are the two bone folders that I use. As you can see, by the aged look of the bottom one, I've had it the longest (since college). The only reason I got the second one (part of the Martha Stewart craft tools) was because the first got lost for a while. I like the curve that my newest bone folder has (offers a comfortable hold) but it isn't absolutely necessary. They both work just fine for me.
If you need to score a piece of paper but don't have a bone folder the dull side of a knife or pair of scissors will do the trick. If you use the latter, you have to be careful not to press too hard because the paper may tear.