There is only one border crossing between Mexico and Belize and this is just South of Chetumal.
Before crossing the border make sure you fill up with Fuel as it is way cheaper in Mexico than Belize. Belize is over US$5 per US Gallon for Diesel compared with under US$3 per US Gallon in Mexico. We actually met a Belizean truck driver filling up just before the border so we knew it was the right thing to do.
As you approach the border you can see a bridge in the distance. The river is the actual border between the countries. But first you have to check out with immigration on the Mexican side. This is a small shack at the side of the road just before you come to the border station. You need to pull in alongside the road next to this shack (there isn't much space to park but someone should show you where to go). You need to show your passport here and hand over your tourist card. We weren't asked for any money but some people have been asked to pay US$20 to exit the country - this isn't a real charge so always ask for a receipt.
After this you should have a stamp in your passport. The next step is to drive through the border post and pull in immediately on your left. This is the Banjercito where you need to get them to cancel your vehicle import documentation. Only do this if you don't intend to drive through Mexico again in the next ten years. They should refund your deposit if you gave one - we didn't. Someone will come out to your vehicle and take digital picture of it and should remove the sticker on your windscreen too.
That's the Mexican formalities done with so you then get back in your vehicle and drive over the bridge.
Almost immediately the other side of the bridge you should see a small white building with "Atlantic Insurance" written on it. You should stop here and go inside and buy liability insurance for your time in Belize. We paid US$25 for 1 month of insurance (way too much but 1 month was basically the same price as 2 weeks). Liability Insurance is mandatory in Belize.
After this get back in your vehicle and drive down the right hand fork in the road (to the left is the free trade zone with lots of duty free shops). You will drive past a casino on the left and then soon after this you will see a sign saying something about Quarantine just by a road going to the right. Don't got the right but immediately after this fork in the road is a small hut. You must stop there to get your wheels sprayed with disinfectant. We paid US$6.50 for this privilege. You will receive a receipt for this which you will need to enter Belize. We didn't stop here and were sent back before we could go any further.
A bit further on the road bends to the left - you should go straight on and park in small car park on the left next to a building.
Next to the car park is a fence and behind this fence are the moneychangers. You have to do business with them through the fence but they seem fairly honest and the guy we dealt with was fair.
The immigration building is just next the car park and you need to go in here and first go to the immigration desk. This was really quick for us - the immigration officer asked us how long we were planning to stay and we got our entry stamp straight away.
When we arrived they were celebrating International Customs Day or some such which involved a Customs officer DJ’ing load reggae music. So loud that you could hardly hear inside the immigration building. No stern faces here – they do things differently in Belize !
Just past immigration is the customs desk and you need to visit this (on the something to declare side) and get a temporary import form for your vehicle. You also get a stamp in your passport which links to your vehicle. This only took a few minutes and the documents we needed to show were our registration form, our insurance and passport - that's it - no copies of any document were required.
At this point you turn round and walk back out of the immigration building the way you came - although any passengers need to keep walking and meet the driver the other side of the border.
You pick up your car and drive through the border. At this point you will have to show your import documentation and your proof of being sprayed. Importation of fruit, vegetables, meat and alcohol is not allowed so you will be asked about this. I said we had nothing but a couple of beers and this was accepted as fine and I was allowed to drive through and pick Sarah up. At this point we were in Belize.
The whole process had taken about an hour. The great thing about everything that happens on the Belize side is that it is in English !