I do this whenever I have a short sleeve cotton shirt I'm washing and drying. It saves me time on ironing.
Step 1: Materials: Cut 2 Pieces of Cardboard, That's It!
For my shirt size I did 2.5 in x 8 in. This will obviously vary by shirt size. I would recommend a width of 2.5 inches, so the piece is firmer.
Step 2: Insert Cardboard Into Sleeve.
Do this when the shirt is fresh out of the dryer and still warm. The cardboard will make the fabric flat. Keep the cardboard in until the shirt cools.
Step 3: Once Cool the Sleeves Will Be Crisp and Ready to Wear.
Keep about 10 pieces around while doing the laundry. I will take the pucker out of the sleeves and save on ironing time.