Hi Tracey - the best way to burn fat is by alternating your heart rate - high/low etc. while staying in your target heart rate zone - so not getting too out of breath.
General rule is that you should still be able to speak but not sing!
On a bike - and it really helps toning your bum and thighs - if you do "in the saddle/out the saddle" with a higher resistance - like you are going up hills.
I do this on my real bike - tend to do 12 pedals in the saddle and 12 out and just alternate like that - being out the saddle tends to increase your heart rate also.
Hope this helps and that you are still enjoying your exercise. xx