You traced the shape of the top flap on the second piece of graph paper. Now sketch out an overlapping shape to use as mirror images for a side flap. It doesn't need to come to the middle if the bottom and tops will overlap in that middle section.
The corner should be pleasantly rounded.
The bottom of the side flap is done the same way, keeping in mind an idea of where the bottom flap will go.You only need to cut out one of these side flaps.
Use the side flap to trace its shape on the stiff paper and on what remains of the graph paper. Remember not to trace the flap upside down!
Now for bottom flap. You'll make a mirror image of this and flip it to trace both sides (or you can use a third graph paper rectangle!) So you only need half of the sheet.
Take a look at where the side and top will be meeting and overlapping. Mark a dot where the bottom of the envelope should meet the side. Also make a dot near the corner. That marks where the slant will change, being steeper near the corner and more gentle towards the middle of the envelope. Note that I've shaded the area where the flaps will overlap to help visualize where the bottom flap needs to be. After sketching out the shape, cut it out.
Trace half the shape, flip and trace the other half.
Trace the shape again inside the envelope to help visualize what the envelope will look like. All the flaps should provide enough overlap that there are no open cracks.
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