If you have a score board, it will more than likely have a mark, showing where to score, for an A4 half fold card. Make sure that you put the short edge of the card right up to the score board barrier so that the card is scored correctly. If your score board does not have any markings, then simply measure the length of the piece of card and mark the card half way along at the top and the bottom. Align these lines to the score board ridge and then score. It's important to measure each piece of card as they can vary slightly each time.
Without a score board, you can still achieve good results but you will need to be a little more careful. It is possible to simply fold the cald in half and crease, but this can cause the edge of the card to crack and leave you with unprofessional results. You can buy a 'ball embossing tool' for a couple of pounds but if you've got an old, out of ink, biro then that will work just as well. Measure the long edge of the card and mark the centre line with a pencil at the top and the bottom. Line up the ruler with those two lines and then use the ball tool, or biro, to mark that line. It's best to go over it 3/4 times just to create a slight indentation.
When you have scored the card, turn it over so that the mountain of the indentation faces up and then crease towards you. This means that the valley aspect of the indentation you made, becomes the outside edge of the card- this gives you the crispest fold. This is where you use the bone folder to run down the edge of the card and make the fold extra crisp.
You can buy pre-scored cards that you simply have to fold but these are more expensive and not available in as many colours as the plain A4 card.
This is a very versatile card fold and below are some examples of cards that begin with that basic fold, in both portrait and landscape orientations.