Chelsey is right... the best way is just to come out and say it, preferably FACE to FACE. My mom told me that she was so happy that I didn't tell her over the phone, etc. Try your best not to sound upset or cry (that was my mistake). I know you'll be nervous, but if you feel generally positive about this pregnancy I feel it's important to show it.
Trust me that they will be upset at first... that's why there is no good way to break it to them. But, they will come around eventually and love your child and want to spoil it to death
At least mine did. Just don't put it off. My mom wish I had told her much earlier.
It's great that you have a plan to share with them to show that you're being responsible about this. Good luck, and keep us updated!