Another quick and easy baby craft. Again this was mostly improvised, so the pictures tell the tale. Feel free to alter as needed.
Supplies: Sewing machine, pins (yup, even used 'em), scissors, rotary cutter with mat and ruler (these are optional, but I like them for straight lines) and outgrown onesies. I found that the last outgrown set fit her head pretty well, though your mileage may vary.
Step 1: Turn the onesie inside out and cut straight across the bottom just above where the crotch bit starts.
Step 2: Fold the onesie in half the "long" way and cut an arch just below the neck and sleeve seams. Here you can make it as "slopey" or as "flat" as you want depending on what style of hat you desire. If you are quite clever and measured you little one's head, you could cut inside the side seams to make the hat narrower if needed.
Step 3: Sew around the top curve. I did a straight stitch about a 1/4 inch in and then did the raw edges with a zig-zag stitch. The edges don't need to be finished, but it makes it all a little neater. If you cut the hat down to size, obviously just sew around any "open edges" in this manner.
Step 4: Fold the hat right side out and fold up the bottom edge roughly 1/4 inch, pin in place then sew around with a zig-zag stitch making sure to catch the raw edge as you go around.
Step 5: Roll up the hem as desired and tah-dah, its a hat...for your head!
I made two to go with the T-shirts also crafted out of the outgrown onsies. Just cut off the crotch bits and finish the edges with an overlock or zig-zag stitch.
My adorable little model showing one off: