From San Ignacio you can take a collectivo to the border for 5-7 BZD. Once you get to the border you have to get out of the car and walk across the border. There are two separate lines. Unless you’re a Belizean citizen you’ll have to stand in both lines. The first line (the one to the right) you stand in is to pay the exit fee (40 BZD in total, roughly $20 US). Once you have your receipt saying that you’ve paid the exit fee you can then move to the regular line (the one to the left) to pass through immigration.
After you’ve cleared immigration you’ll exit the building into a wide open space. The large building a 1-2 minute walk to your left is the Guatemalan border. You HAVE to also go through their immigration or it could be extremely complicated for you to leave the country. This time you wait in the line that is for those ENTERING Guatemala. You’ll have to pay (20 catzales to enter Guatemala, roughly $3 US). Finally, once you’ve done that you can continue on into Guatemala. Cross over the bridge to catch your bus to your next destination.
You have now crossed the border into Guatemala.
IMPORTANT: For those of you trying to take an overnight bus to Guatemala City the sooner you buy it the better! The overnight bus leaves at 7:30 pm, but if you can, see if your tour guide, someone at your hostel/hotel or kind stranger knows anyone who can buy it for you and you can pay them back either the day before or early that same morning to guarantee you a spot on the bus. This does not mean that you can’t buy your ticket at the last minute, but just that it’s safer to know you’ll definitely have a spot on the bus before crossing the border and finding yourself stuck without a bus or a place to stay.
NOTE: If you’re going somewhere closer like Flores, these buses leave much more frequently so you don’t have to worry about buying your ticket in advance.