To do it properly, you really need to use an extractor. Our method would be to thoroughly wet shampoo the carpet using a products like Autosmart Bio-Brisk, working the shampoo in gently using an interior carpet brush (Meguiars All Surface Interior brush is ideal). By foaming up the shampoo you can easily lift deep seated grime. The next step is the important bit - you need to suck up the foam and as much moisture as possible using an extractor (also known as a wet n dry vacuum cleaner). If you fail to do this and leave the carpets overly wet, then you risk mildew setting in. If you don't have the right gear, it would be best to pay a good valeter/detailer to do this job for you. Other options include buying a cheap extractor, or hiring one, as suggested above.