A louage is basically a big taxi. Most towns have a louage point - a noisy and usually fume-filled parking lot where the white and red vehicles wait until they have as many passengers as possible before setting off. When you arrive speak to one of the drivers and tell them your destination. He'll wave his hand vaguely, try to head in that direction and you'll probably need to ask again to be told which vehicle to get in. You are meant to wear seatbelts, the driver usually doesn't although the signs over the motoway says he should. Locals usually have a word with the driver to arrange to be dropped off places other than a town's official louage station, my advice would be to stick to the main louage station and alight at the proper terminus.
Somewhere close to the final destination, your fellow passengers will start paying. There are usually three rows of seats. Help out by passing money forward to the driver who will then risk life and limb getting change from his trouser pocket whilst negotiating the busy traffic.
We travelled from Hammamet to Tunis and paid around 5 Dinar for the pair of us - less than half the cost of the train (although that's pretty cheap too)There are two main louage stations depending on whether you are coming from/going to destinations north or south of Tunis, but both are pretty central.
Trains to Tunis from the coastal resorts only operate during peak commuter times so the louage has the added advantage of running all day. Bear in mind that the driver will only go when he feels he has enough passengers. If it seems to be taking a while to fill up, you and your fellow passengers may be able to negotiate with the driver to pay a little extra to compensate for empty seats if you are keen to be on your way.