A Fold
With most papers (170 gsm and under) you can fold by hand, easier with the grain than against it.
To be precise with a fold, a mark can be made with a blunt instrument, i.e. a non inking biro or rounded metal pointer, as long as the fibres are not disturbed.
On heavier board an indent is required. This can be produced by making a trough and pushing the board into it or better still, fixing an upright rule in a block (a Maun metal ruler is a good alternative) with the board on top and pushing down along the rule with anything that has a groove in it, e.g. the side of a pen or a notched piece of hard wood.
This indent organises the fibres of the board so that they will not be stretched when the board is folded. Fold the board with the indent channel facing outwards with the bridge of the board forming the excess fibre pocket at the back. This particular method is the best, especially if glued layers have been applied.
A Score
An alternative way of creasing. It has a clean precise finish, but is only half as strong. With a craft knife or scalpel blade, make a cut in the board no more than half way through.
A Perforation
Perforating the board by making a series of holes or slits will weaken the fibres enough to enable the board to be folded (and intentionally torn, if the holes are close enough). The finish is not that attractive and weakens the board considerably. However, a weak fold is often required for a flattened crease to pop back up easier!