There are a couple of ways you could go about it. It depends on if you want to use tips or if you want to use forms.
For tips I take a clear tip and use the acrylic as the bonding agent to the natural nail, that way it's giving a little length and base to work with, then I was another tip, usually the white tips without a well and also use acrylic to bond onto the clear tip. To disgiuse the nail underneath the clear tip I use and opague color for the nail bed. (E.g. Tammy Tayloy's pinkest pink.)
Or...
If you want to use forms. Place the form close to the nail and put a small bit of cuticle oil if any skin is showing. I extend the nail bed with my opaque color again, and then you can 'pretend' your new sculpted nail bed is the 'real nail' and sculpt a tip or glue a tip on. They work as usual.
I think both methods for me work well, i started doing enhancement on a girl at my school who's going through a custody battle, and become an avid nail bitter. When we did her first set i used clear tips over maybe 1/3 of a cm of actual nail left on the bed. her nails have been able to grow out now, and are quite beautiful now.