Best Way to build your cardio? Long Distance Running. Period. This can also be referred to as "Steady State Cardio" Essentially you want to keep your heart rate up at a sustainable rate for a long period of time. 20 mins isn't a bad place to start for most people, I know Professional Boxers that run 3 miles a day. At a 10min mile pace that's a 2 mile run, the pace you run at is also important. You want to keep your heart rate up, but you don't want to kill yourself doing it, you should be running at a pace you can hold a conversation at. Biking, Rowing, or an Elliptical can also work for "Steady State Cardio" I just find running the easiest and most effective to manage.
What about HIIT training? The Germans first introduced this concept, of high intensity interval training. They would raise heart rates to about 85-90% of capacity for a short period of time, and then cool down with a walk or light jog. While they found this does work for increasing cardiovascular capacity it wasn't as effective as "Steady State Cardio" in fact they would often do HIIT training AFTER long distance running.