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I used my Martha Stewart scorboard and marked the center of a 12x12 paper and then using a stylus scored a line straight down the 6 inch mark about 2 inches.
then using a 12 inch ruler you want to measure exactly 12 inches on an angle from the bottom left corner to the center mark. Remember you will only be going up to that center mark (about 2 inches below the top of the paper). Now you want to do the exact same thing from the right bottom corner up to that center mark. You will be cutting out your triangle as shown.
Yours won't have the score lines yet for folding, but it is the next step. Put one of the straight 12 inch sides along the board and score at 5 1/4 line and on the 7 inch line. You will turn the triangle and do all three sides the same way. You will end up with a center triangle and 3 side wing triangles.
You will be making a mountain fold at the 5 1/4 fold and a valley fold at the 7 inch score lines.
Then just fold it up, fitting the flaps together to hold, kind of like closing the flaps on the top of a box.
To decorate let your imagination have fun. These were fairly simple but you can get as detailed as you choose. I used the extra paper from where I cut my large triangle and cut 3 strips 1.5x6 inches between the folds. I cut a 6 inch square of cardstock and 3 smaller 3.5x3.5 square to form the contrasting triangles using the exact same method as I did for the 12 inch one above. For the outer focal point, make sure when you adhere it to the outside that you only glue it to one triangle so that you can open your card.