I've never seen a tutorial for them or tried it myself but why not just give it a go ? Do you have any hollow mandrels ? Wrap some glass around the end of the hollow mandrel to make a hollow bead then gently blow while spinning the mandrel. Once you have the size hollow you want, pull the end of the glass away from you using some hot glass on the end of the color rod you used to make the hollow I'd think. This is just off the top of my head but makes sense to me anyway. lol ! I have no idea if this will work as I said, I've never tried it but now that I've thought about it I just might !
So let us know if you try it and post your results.