Fold your square in half along both the diagonals but make sure not to crease your paper all the way along. You just want to press down and crease your paper in the middle.
Open your paper out. The folds should have made a small 'X' in the center of the paper. Using the 'X' as a guide fold 3 of the corners in to meet in the middle. Your paper should now look like a small house with a triangle roof on top.
Take the bottom of the paper and fold it in half to meet the bottom of the triangle. Now is a good time to run a bone folder or the flat side of a ruler over the creases to make the folds are nice and crisp.
Fold the top triangle down to make the final flap.
Trim the edges of your envelope in your chosen washi tape and stick down the 2 sides. I found it gave a much neater finish to trim the inside of the closing flap first and then stick the sides down afterward. Wrap the washi tape around both sides.