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BEST. INSTRUCTABLE. EVER.Nothing to purchase, no moving parts, and you can use it right away!
Start by folding the sleeve back the same way (180 degrees), then fold it again down the length of the shirt (90 degrees). If you do it right the two sleeves lie parallel to each other. Hint: do this just before rolling the shirt.Another hint about the whole instructable: just do it with the top of the shirt toward you, and the cuff end away. Then you don't have to turn the half-folded shirt around to roll it.
How to Ranger Roll a T-Shirt
This turns out to be an awesome instructable. Kudos to @yonatan24 for writing it and, more importantly, being open-minded enough in the comments to understand the things he did wrong and recognize better solutions. Thanks also to @thestip for the detailed "instructable comment" (though he might have been a little more polite) and @diy_bloke for providing a lot more technical and practical background. In this case, the comments ARE the instructable!