When we went 2 years ago, I used a travel agent in Guatemala who did a fantastic job of making our arrangments. We were picked up by a van from the airport that took us to the Jungle Lodge in the park. A guide was provided and we took a 3 hour afternoon trek and a 3 hour early morning trek through the park. The guide we had was fantasic, very knowledgable. I would highly recommend using a guide. If I remember correctly, guides in Guatemala have to go to school for quite a while to be allowed to work as a guide. If you want, I can dig up the name of the agency we used.