Hi. I was in Belize last November, and we took the Dangriga bus from Belmopan to get to Caves Branch. (We were coming from San Ignacio.) It was simple to do and inexpensive; just ask the driver to drop you off when you get on. Caves Branch has a small bus stop with roof and a large sign, so you'll know you are in the right place. The walk in to Ian Anderson's is along a dirt road and is a half-mile to mile long.
When we left, we hadn't reached the bus stop when we heard the bus coming. We started running down the road with our backpacks on but saw the bus cruise on by. However, the bus driver had seen us, stopped, and backed the bus up to pick us up, the kind soul!
Have a great trip. My wife and I loved ours.