For this step you will need your bound envelopes and metal ruler.
Based upon the size of your envelopes, you want measurements for five pieces for your cover elements. Two pieces for the front and back cover and three for the spine.
Your front and back pieces should be a quarter inch longer than your envelopes vertically (picture 1) and a half inch shorter horizontally. The envelopes I used were 4" X 6 7/8"; so my cover dimensions were 3 1/2" X 7 1/8".
Measure the unbound spread of your envelopes (picture 2) to determine the size of your spine. This needs to be much larger than your actual bound spine of envelopes because the plan is to fill up the envelopes with stuff and this will give them room to expand without unsightly cover bulging. In my case, I chose to make the main spine 3/4".
Your spine pieces are the same height as your cover pieces, in my case 7 1/8". The wings should be 1/2". This measurement makes up for the 1/2" we cut off from the cover pieces. The final dimensions for my spine were one piece at 3/4" X 7 1/8" and two at 1/2" X 7 1/8"