After you install your strings and have tuned them to pitch, you can start the stretching process. Start by placing your fingers of your left hand, nails down under the high "E" about four inches from the bridge. Place the fingers of your right hand beside your left hand and press down about 1/4 of an inch. Move both your hands towards the headstock about and inch and repeat the procedure until you have reached the 2nd fret. You can get an idea from the animation below.

This procedure should make it almost impossible to break a string. By just pulling the strings with your right hand alone, it is almost impossible to judge how much you are pulling and you WILL break the plain strings no matter how careful you think you're being. No really, we've tried, it's very frustrating.
Retune the string back to pitch and move to the "B" string. After stretching the "B" string, retune it and repeat for the rest of the strings. After stretching all six strings, repeat the procedure three times. This should take about 20 minutes to completely stretch all the strings thoroughly so you might want to do it while watching T.V. or something to occupy yourself since it is a bit repetetive.
Never start this procedure within a half an hour before playing as guitar strings are a lot more elastic than you think. As the strings start to pull back, you will actually notice your pitch rising a bit. It's best to restring your guitar, especially classicals, the day before an important gig. They need a day to stretch out. The audience wants to hear you play, not tune your guitar. Remember to add a bit of graphite from a pencil in your nut slots to reduce friction to help you keep in tune. Good Luck!