I am definitely not the originator of this idea; I came across it awhile ago on the internet but don't remember where. However, it is so useful that I hope it's okay to reshare it, in the interest of helping others struggling like me to get their patterns back into the envelope.
I usually trace my patterns. I like to preserve the original pattern, plus I just find it easier to make any alterations or notations on my traced pattern piece. I use a sharp pencil and thin tracing paper that I ordered here. Sunni at A Fashionable Stitch has an explanatory tutorial on How to Trace Sewing Patterns, if you want more information on that. She also sells the necessary supplies.
After I finish tracing the pattern pieces I will be using, I lay the tissue sheets out flat on top of each other and fold them until they are slightly narrower than the length of the pattern envelope. (This will work even if you cut out the tissue sheets themselves. Just lay them out as flat as you can and start folding.)
I have a clear ruler that is slightly less than the width of the pattern envelope. You can use anything you have that would be the appropriate size. I lay this ruler on the tissues and start folding inward.
Here are the sheets all folded up:
And here they are nicely stuffed back in the envelope: