I prefer doing mine on the stairmill. I will do intervals of level 15 or 16 for 30 seconds followed by level 7 for 60 seconds. I will do that for 15 minutes followed by 15 minutes of steady state at around level 7 or 8. If I feel like changing it up I will do 15 minutes straight on level 12 followed by 15 at level 7. I know thats not HIIT but it still has the same EPOC effect. Regardless of which method I use I aim for a HR of around 160-170 during the high intensity portion followed by 130-135 for the last 15 minutes.
I usually do this PWO or just on its own in the afternoon on days I dont weight train.