Get a timetable from Waterloo Train Station and check times for trains to Salisbury. Timetables are also available online. If you have access to a computer and the Internet, check there instead.
Choose a day and time that is convenient for you. Keep in mind that travel time, one way, to Salisbury is about 90 minutes, so you will need a whole day for this trip.
Purchase train tickets. You can buy them ahead of time online or in person at Waterloo train station. Tickets can be purchased in advance or on the spot.
Arrive in Salisbury and arrange for a taxi to take you to Stonehenge or find the local bus schedule. Buses do run regularly to Stonehenge from Salisbury and are cheaper than a taxi.
Tour Stonehenge and listen to the audio tour on headphones. Admission to the monument is reasonable and there are discounts for children, students and seniors as well as a lower family rate. The headphones are included in the price if you would like to listen.
Browse the gift shop for a great souvenir. Stonehenge is such a unique place that you will not want to miss buying a one-of-a-kind souvenir.
Take the bus back to Salisbury to catch the train. Make sure you have allowed enough time to make the return train.