score at 1,2,5 and 6cm, turn the card and repeat on other three sides.
Cut the corners as shown, make a further cut so the little corner square is only attached on one side.
make diagonal cuts from the 5cm score line.
Glue the corners, fold on the inner score lines
You can see the score lines on the finished frame, this doesn't matter if you are covering it but if not then you will have to make your score lines starting 5cm down from the edges... more difficult but not impossible.
Cut another piece of card to fit in the aperture.To finish cut a square of card to fit the back, this makes a very sturdy frame. you can cut a strip of card or paper to hide the spaces in the corners... sometimes I do sometimes I don't.
You can make these any size, square or rectangle.