This site was very easy to get to, but you need to coordinate your trip with the hours that the hand ferry is in operation. It opens in the morning, closes for lunch, then the guy returns for the afternoon. Try to make sure you made the ferry by 4pm.
To get to the ruins take the Western Highway out of San Ignacio towards Guatemala. Watch on the right for a ferry. At the ferry crossing there are craft booths along the shore. (If you get to Guatemala, you have gone too far.) Once you cross on the ferry (free) you take the only road to the ruins (less than 5 minutes). There are two gift shops and fabulously clean restrooms before you enter the park.
The ruins were very welcoming. We went on our own and first stopped in the museum to read about the area. From there we walked around the ruins and followed the paths to climb to the top of the temples. The view was spectacular. The largest temple has reproduced reliefs on both sides that are stunning.
The visit took us under two hours as the area was small, and was well worth the trip. We divided the day between Xunantunich and El Pilar. Between the two places we were out for the entire day.