Bus travel in Belize is a cheap and convenient way to travel.

Getting on
You do not need to buy a ticket in advance. All towns have bus terminal, but in many cases it is possible to stop the bus at any point on its route and jump on or off, unless it specifies that it is an 'Express' bus. Make sure to make yourself seen well in advance if getting on along the highways. If you have large bags, walk up to the rear and throw the bags in from the back, as there usually are no baggage compartments on the sides.

Cost
Simply take a seat and wait for the ticket guy to walk up to you and sell you a ticket. Unless you travel through the whole country, the ticket will usually be in the region of 5-20 BZD. Cash only and preferably BZD, even if USD is ok too. The ticket guy might not have much change, so don't pay with large bills.

The buses
Almost all buses in Belize are old US School Buses, so don't expect anything fancy.

Timetables
Timetables are hard to find on the net, but can be found in the terminals or the local tourist office. Buses leave according to schedule. On the main highways there will be buses pretty much every half hour, so there is no problem in for example jumping off at the Belize Zoo and then just walking out to the highway when finished. A bus will probably arrive in no time.

Here are some websites with bus schedules...

James Bus Line schedule can be found at 3 sources:
Hickatee site: https://www.hickatee.com/belize_bus_t...
PGBelize: http://www.pgbelize.com/jamesbusline
The Toledo Howler: http://www.belizenews.com/howler/
Minor differences exist; I don’t know which one is the most accurate.

Ritchie’s Bus Service
Website: http://www.ritchiesbusservice.com/ind...
Facebook: https://www.facebook.com/pages/Ritchi...

There is another travel guide website (not a blog) that contains bus schedules. TA continues to remove the link for the website, but it can be found by entering "BelizeBus" into your search engine.

Bus travel to and from Mexico or Guatemala

Belize's buses also travel to and from the border with Mexico and Guatemala.  In addition, there are Mexico buses (ADO) and Guatemala buses (Linea Dorada) that travel to and from Belize City (generally drop off is only in Belize City, not en-route), to cities in these neighboring countries.  Availability and schedules vary based on demand.