As paper gets heavier, it gets harder to make a clean fold. Some papers get bumpy and rough looking on the spine of a fold. This does not usually happen with lighter or medium weight papers, but as you get into heavier weights like card stock, the folding becomes more difficult. To fix this problem, the paper should be scored on the fold line and burnished after the fold.
Traditionally scoring was done with a piece of ivory called a bone folder. Of course ivory is no longer used. Place a ruler along the line where you want the fold to be. Draw a bone folder along the edge of the ruler like you are making a line. Push hard. It will crease the paper. Fold along the crease with your hands. You can fold either into the crease or away from the crease. Try both ways and compare the results. If the outside spine of your paper cracks after folding, fold the other direction. It should be much smoother. Use the flat surface of the bone folder to rub the fold flat. Practice positioning the ruler until you get it just right. The corners should line up when the paper is folded.
At Paper Depot we have scoring guides in our free customer workroom to help you position the fold correctly. Fiskars makes a great rotary cutter that can have a scoring blade put into the cutting head. That is also available in our customer workroom. Of course we sell all of these tools for those who like tools and prefer to work at home.