In classes that I hold, this question comes up from time to time. So I thought I'd share the answer with you and I added a link back to this post in my tips and techniques section here on my blog. So you will always be able to refer to this post.
What direction should I fold the cardstock after I've scored it?
First you will need to score on what will be the outside surface and then fold the card in a mountain fold so that the bump on the reverse side ends up inside the card. By doing this you will get a much cleaner fold.
Keep in mind, when scoring on plain cardstock there is usually no front or back.
When folding concertinas you should score every second line on one side and then flip the card to score the other lines so that they fold in the opposite direction, this will enable you to fold in valley and mountain folds.
Remember this, when you score using a score tool, you are pushing the cardstock into the groove on the tool.. So as you look at the card stock that you have scored, you will see a dip, that means that score should become a mountain fold.
Here is a shot of a scored card folded with that dip on the outside
If you look at the other side of the scored card stock, before folding, you will see the bump and this will become a valley fold.
Here is a shot of the scored card stock beginning to be folded. The bump is showing and as you fold the card that bump will be in the inside of the card.
Hope this clears this up for you.