Here's a tip on how to sew objects onto your project using a simple guidline made from yarn in a contrasting color.
Note: Embroidery thread is a good guidline as well. Save these yarn stirps, you can use them later.
1. Weave a strip of yarn through a stitch, using the center to help find the middle of the circle below. Then you do another strip of yarn perpindicular to the previous one. It should look something like this.
2. Using a couple straight pins, pin down your parts and then sew them on.
3. When done with sewing, remove the gridline and the result should look something like this:
You could use this on a number of items, like an amigurumi or when sewing on appliques.