Definitely use painters tape. And I've always used a level to make straight even lines. I'm sure a designer will stop and give you some great tips. Good luck with your project. :) Just remember, if using two colors or more, you should do each individually and let dry before re taping. Also, I find it best to carefully remove the tape after you finish painting because sometimes if it dries the paint can pull up. And be sure to push the tape down so it adheres well to the wall for a clean line. Okay...I talked your ear off enough. :) Good luck! :)

There are lots of online tutorials. Best bit of advice I can give is to use a good quality painter's tape.

You have received some good advice, just wanted to add that stripes look lovely but a lattice pattern can be very pretty too.:)

I have done this many times and the best technique that I have found is this: Mark your stripes using a pencil and a leveler
Burnish the edges of the tape well
Paint over the edges of the tape with the color that is underneath the tap. This step is what will give you the straight clean lines because it seals the tape and if there is bleed-through, it will be the same color that is under the tape...genius right!
Now, paint your stripes and remove tape asap, preferably while the paint is wet
Finally, if there is any bleed-through with the contrast use a small canvas paint brush to correct that o-crap error and they will be PERFECT!
Good luck and congratulations on the baby!

Have you considered removable paper.....maybe on just one wall?