I used to do this years ago. Basically between weight lifting sets, while you are resting, you do something like jumping jacks, step-ups, etc. to keep your heart rate up. You do it for like 30 seconds to a minute. That way if you do 3 sets, you have done 1.5-3 minutes of "cardio". At the end of a full workout of say 15 sets, you would have got in 7.5-15 minutes of cardio in.