The total length is just short of an A4 sheet of card, though this could easily be adapted to fit, by reducing the height of the mountain fold. To make it all easier here are the measurements.
Two pieces of card to measure 12 cm high and 15cm wide.
A third piece to measure 9cm high and 15 cm wide.
score at 2cm , score at 4.5cm, score at 7 cm.
With glue adhere the first 2cm to the left side card and the last two cm to the right side card.
Bend the scores to have the centre at 4.5 a mountain fold and the outer two folds are valley folds.
cover it all in nice paper. If for yourself, I didn't bother tidying the inside, but if for a gift adhere more paper to tidy the inside. Mark with a pencil where to place the holes for the rings or ribbons. Punch in two holes. Fold upwards, add rings then start making your monthly pages. . Here are my February and March pages. More will be added as they are created. The details of each can be found on my main blog, labelled Calendar.