At this level the dog should now know the heel command well and be walking on a nice loose lead. We are now wanting to transition that heel work to being off lead. If you find that a little daunting you can use a heeling lead. Put the heeling lead on and remove the slip lead. Just let the heeling lead fall and then go back to basics as you did with your on lead heel work. Change pace, change direction. Remember to reward your dog. Take to different places and different times of the day to ensure it is 100% reliable.