It's very easy!! :)
You just need to enter the cities on www.busbud.com, the dates and the number of passengers and click on Search for buses.
(If you want a one way ticket just leave empty the box marked as Return date)
First you will be able to see all the options for your outbound trip:
If you click on Show Details you will be able to see more info about your trip, departure and arrival addresses, trip duration, stops and transfers, terms & conditions and more.
Once you've made your choice just click on Select. If you are taking a round trip you will now be shown all the options for your return trip. Your outbound trip is shown at the top of the page and the options for the way back are at the bottom.
And then you're off to the checkout page. On the right side of the screen you will see how much you will be charged.
Enter your name and double check the spelling on your email address! :)
After you enter your credit card details, you're all set. Just click on Confirm & Book