The original directions call for a square of 8 7/8 x 8 7/8 paper which means you can't use a standard size piece of paper and gives you a lot of waste of some good paper. I did a little playing around and figured out how to make an A6 (card size 4.5 x 6.25) out of a 8.5 x 8.5 inch square!
Align left edge of your 8.5 x 8.5" square at 5, punch and score
Rotate paper counter clockwise and align first score line with score guide
Repeat step 2 and 3 and then your paper should look like this picture. You can use the corner rounder at the back of the board to round your corners at this point. I usually only round the corner of the top flap since the others won't be seen.
I like to "tuck" the bottom flap to make it go straight across because I just think it gives the envelope a neater look. You can see a faint line where I make a slight score mark so I know where to fold the little flap down.
Just use your favorite method to glue or tape your envelope together and there ya have your finished A6 envelope out of an 8.5 x 8.5" sheet of paper!