This is one of my favorite projects thus far and I think it is just the cutest thing ever! You will need a sewing machine for this project. The onsie itself I bought in a pack of 4 for about $11, so it’s a little less than $3. I bought a 1/4 of a yard of fabric for the tie, which was probably more than i needed. I also had to buy some iron on bonding paper (which you can find in the fabric section and you only need a 1/4 of a yard of this too).
The bonding paper is basically used to fuse the two fabrics so that they can be easily sewn together. Cut out a rectangle of bonding paper a little larger than the dimensions of the tie shape. Cut the tie fabric out to these same dimensions. Iron the fabric onto the bonding paper (wrong side of the fabric to the adhesive side of the the bonding paper). Once the tie fabric is bonded to the paper, you can easily cut out whatever design you like. Since it is fused to the paper it doesnt fray when cutting. For this project I just cut out a general tie shape that would fit nicely on the size of the onesie. Make sure you mimic the curve at the neckline. Once you have your shape cut out, you peel off the paper part of the bonding paper (this leaves the bonding agent stuck to the back of the tie fabric). You then just position the tie where you want it and iron it on. Now the two fabrics are bonded together, but this will not be very secure, so you need to sew the two together. I just sewed a border around the entire shape to secure them togethe (if you want to get fancy you can even use a zigzag stitch!).