I've done it two ways....
1. Every 30 seconds, I bump the intensity or the speed up on whatever cardio machine I'm on. I go up and down every 30 seconds, using the slow 30 seconds as my recovery time. Sometimes I do this method in 60 second increments.
2. The second way I do it is to listen to a podcast from Stin (free on itunes) where she takes you through a 20 minute HIIT routine. Two minutes of warm up, then she takes you up a 'level' each minute. You decide what your levels are but it's the same premise of increasing speed or resistance. So, two minutes warm up, 1 minute each at level 6, 7, 8 & 9 then repeat a total of 4 times ending with 1 minute at level 10 and a cool down.
At this point, I can't say it has positively impacted my weight REMOVAL but I can certainly notice it in my fitness level. The long slow burn way of doing cardio is fine - but I get so incredibly bored. With the HIIT sessions, even if I do 2 of them, they are more interesting and I'm amazed at how much more I can do, intensitywise.