When I extend nail beds I tend to use the reverse application technique.
I apply my cover pink or custom blend opaque colour where I want to extend the nail bed to and create the smile line, and then blend the product back down the nail. I then perfect my smile. Then I apply my free edge colour finishing off the smile, then a layer of clear or transcluscent pink over zones 2 & 3 to give me my structure and apex.
Most companies offer advanced training courses, for these sort of advanced techniques, maybe you could look into that, it definitely helps to be shown how to do this rather than reading about it.
HTH