I was just reading about this last night...The best method that I read was to use a tampon applicator. Take the cotton out, put some tape over the opening of the end that is inside of the larger tube, or you can use a cotton ball to stop it up. Place it back inside of the larger tube and fill the larger end with the yogurt. Push it in just like you would do a tampon!
I also read that you should make sure the yogurt is white, all natural, with live acidophilus cultures, unpasteurized, and without preservatives added. Unfortunately, I don't really eat yogurt, so I don't know what brand that would be.
Hope this information helps.