the tummy to tummy hold is very similar to the "hip carry" when used with slightly older children-- here i am showing this carry with a newborn. it is wonderful for babies who like to have their head up because of curiousity or (hopefully not) reflux. my son was in tummy to tummy in my ring sling most of the first few months of his life!
if you have a young baby you will make a pocket with the sling and put the baby down feet first, facing you......with his feet curled up under him in the pocket. then just gather the excess fabric around to the rings and pull the sling tight... you can have the fabric up behind the baby's head to give head support for very young babies.
for older babies you can put their feet through the sling as shown on the 'hip carry' section. remember to have their bottom below their knees (have them sitting down in the sling). wrap their little legs around you with them in front of you so that you are "tummy to tummy"!