Use A Scoreboard
Scoreboards are one of the tools that belongs in every papercrafter's basic kit, and I highly recommend using one every time you want to fold a piece of paper, especially card stock. Not only does a score board allow for exact measurements without the hassle of rulers, pencils and markings, they also help to press the paper fibres into neat creases preventing most of the "cracking" that can occur when you fold a piece of unscored card. There's a very obvious difference between a piece of folded card that has been pre-scored and one that has not.
Many scoreboards have additional markings on the back for creating envelopes and special folds. You can often find tutorials on the internet for other decorative finishes using your scoreboard including borders, faux wood planks, and embossed patterns. Flip yours over to see if there's anything you've missed!
ADDITIONAL TIP: Don't be shy about adding your own measurements and guides for sizes you use regularly. If you look closely at my scoreboard, you'll see that I've used a permanent pen to add markings for A5 and A6 sizes. I do this on my guillotines, trimmers, rulers and other tools too!
