These are plastic yo-yo forms you can find in multiple sizes in most fabric stores. I'll say it again - you don't need a form to make a yo-yo - but it sure makes things easier. You take a piece of fabric and snap it between the two sides of the form, like this:
There are two distinct advantages to using plastic forms. The first one is this: you don't need to cut a perfect circle before you pick up a needle. In fact, you don't have to cut a perfect circle at all. Using the plastic form guarantees a uniform size even with odd shaped scraps you see above. You don't have to cut the circle until after the fabric is firmly held in the form.
Once you snap the fabric into the form, it's time to start sewing. Here's the second advantage. You don't have to worry about sewing perfectly spaced uniform stitches. The plastic forms have little notches in them so you can sew in and out of the notches to get perfect stitches. It's a great activity for kids.
Once the stitches are all the way around and you cut out the circle, leave the needle in place and remove the plastic form. You will be left with this:
Pull the thread evenly and watch the circle come together. Tie off the end and you have your first yo-yo.
I really want to make some of the cool projects you can see if you look at the Google images, but I haven't made nearly enough yo-yos for that. However, when I was recently making a little dress, I decided to use two different sizes of yo-yos with some little pearls I bought as an embellishment.
Here is the whole bodice. The dress is coming soon - a short tutorial on Straddling the Gap and the actual dress in my Etsy store, Pretty Pretty Cindy