this tutorial here... apart from the score-lines are at 1,2,3 and 4 cm on all 4 sides.
Edited to Add - due to the popularity of this card format and the making some other variants .... I wrote up the full tutorials for you... Imperial (inches) and an alternative doubled version here ( with link to Metric too). Or straight to Metric here.
Ends up as a 13cm Square - or 5 1/4" Square - card. Has a totally different look with the narrower frame section at the front.
I personally still prefer the wider frame for a card - but this had to be done!