Normally at the top left hand end of the blind you should find two round plastic circles with a slot in the centre, use a suitable sized screwdriver and press the buttons in and tighten in a clockwise direction for 3 or 4 turns and ease the button out without removing the screwdriver(if you remove the screwdiver befor they lock in place they will completely unwind)
If the initial tension is not enough, repeat again until you are happy with the tension, BUT , do NOT over tension or you will damage the blinds.
You may have to release the top cassette and move it towards you to gain access to the adjusters