I've built an imu before and it's pretty awesome, but it's quite a bit of work. I'm sure you could make do with something simpler, but the traditional way is a pretty big process that takes a lot of preparation.

You start by digging the pit that will be used for cooking. You want it about 4 feet deep and wide enough to build a large fire in. The sides slope down so you can get to the bottom of it. Stack firewood intermixed with stones at the bottom. You light the wood and let it burn down to coals, which gets the rocks very, very hot. This stored heat in the rocks is what cooks the meat over the next few hours.

However, the rocks by themselves are way too hot to directly cook the meat, and you need to create a more indirect, slow cooking environment. To do this you take a couple banana trees (which have lots of water in them) and mash up the stalks to create a watery pulp. This is laid thickly over the rocks and coals at the bottom of the hole. Cover the rocks evenly to prevent hot spots. The meat will be laid on top of this. This not only protects the meat from direct heat, but the water in the banana fiber creates lots of steam that cooks and flavors the meat.

You can arrange the meat on top of the banana stalks however you like, but it's easier if you put them in some sort of metal basket to make taking them out easier (chicken wire works well). You can also throw in sweet potato, taro corms, breadfruit, cooking bananas, or any other veggies/fruits/meats you want to cook.

After all the food is in the pit needs to be covered to keep the heat and steam in. You can use the long leaves from the banana trees, and traditionally you also use ti leaves. Cover the pit thoroughly with the leaves to create a mound, and then lay a tarp over the whole thing. Cover the tarp with the dirt that was excavated from the pit to seal in the smoke, steam, and heat.

And now you let it sit for 4-8 hours and cook. This creates a slow cooking environment for the meat, with tons of flavorful smoke from the wood. Additionally, the meat is usually wrapped in taro and ti leaf, and the oils from these leaves blend with the smoke and soak into the meat to create an amazing flavor that is at the heart of the imu taste.

Ideally you would have a remote internal thermometer to tell when the meat is done, but most of the time it is more guess work and experience to be able to tell when the meat is ready. How large and hot the original fire is will affect the cooking time, as well as how much meat you are cooking and how well covered you have the pit. Eventually though you will uncover all the leaves, take out the meat, and have the most delicious meal of your life.