Using a plain onesie (I buy them a bit big, as the sizing is always too small, for Gerber anyway), measure and trace a shape or letter of your choice onto fabric. Cut it out and sew the edges back.
Center width-wise and place a bit high on the shirt (so if the babe is wearing pants it won’t be covered). Sew through the middle and double stich around the edges.
We had a last minute shower last week so I didn’t have time to make a blanket, but this was pretty quick.