I recently made a tutu and onsie for a neighbor friends daughter who just turned one. My friend has cancer and this is her third daughter. Since she has been going through chemotherapy she has been really tired so I offered to help her out (everyone needs a fabulous first birthday party in my opinion) and so I made the outfit, a bib, cupcakes and chocolate dipped pretzel sticks as my gift. Here is the onsie and tutu I made her.
Rolls of tulle (in whatever color you want) they sell these at Joann's and Michaels in the wedding section. You can use a bolt of tulle but then you will have more trouble making all even cuts.
A piece of elastic: Measure around the waist and subtract about 2-3 inches. Sew a circle ( I create an X with stitches and a box around it to make sure it is secure.
Measure the length you want the tutu and double it. This will be the length you need to cut all of the tulle piece. Place the elastic on your thigh and begin by placing a piece of tulle under and around the elastic and tie a standard knot. You can use one color of tulle or as in above I used two. Pink and Yellow. The tighter you put the knots the "puffier" it will be. You can trim the back of it with a ribbon bow/flowers or just leave it plain as above.