You should not attempt to have your dog heel off lead until it is practiced at heeling on lead.  Once you are confident that your dog understands the principle of heeling, try putting the leash in your right hand and putting a lot of slack in the leash.  This might trick your dog into thinking it is off lead and you can test your dog's reaction.  Some will try to bolt away, in which case you quickly correct that idea.  Some will continue to heel along calmly.  Those dogs are ready to move on.

Begin off lead heeling in an enclosed environment so that if you lose control of your dog, you don't also lose the dog.  I recommend using a high collar, or a martingale.  Make sure the loop of the collar is up nice and high on the back of the dog's head, and tie something to it, something you can easily grab hold of to correct the dog:  a small scarf, a piece of cloth or rope, or something of that nature. 

Give the heel command and step off with your left foot as you normally would.  Now, even though there is no leash, do not lower your standards or your expectations for your dog's performance.  Do not allow your dog to lag behind, to lead, or to veer off in its own direction.  If this happens, correct your dog by pulling on the object attached to its collar in the direction of correction.  If the dog is ahead of you, pull back on the collar.  If the dog is lagging, pop the collar in the forward direction.  Be sure to time your corrections so that they happen as soon as possible after the dog disobeys the heel command.

There is nothing that says that once you take a dog off lead, you are stuck that way.  Try off lead for only a few minutes a day.  Keep your dog from getting cocky.  If your dog disobeys, throw that leash right back on the collar!

Some ideas for refining the heel:  While heeling, travel in a tiny circle to your left, without actually moving.  You turning around in the same spot in the counterclockwise direction will force your dog to actually back up in order to stay in heel position.  This is an excellent way to make sure your dog knows what “heel” really means.  This is a great drill to use for dogs that want to lead you.  Try also turning in a tiny circle to the right.  This will force your dog to take the tiny circle with you, also reinforcing the the meaning of “heel.”  This is a great drill for dogs who like to lag behind.