There are lessons, tips, links to examples, etc. for making fruit canes (from polymer clay) on this page at my polymer clay "encyclopedia":
http://glassattic.com/polymer/miniatures.htm
...this page also has miniature 3D foods, including fruits, but I've put the fruit "canes" here too...click on FRUITS, under "Foods" but notice which are canes and which are 3-dimensional

These pages have much more info/lessons/etc though on making all kinds of canes from polymer clay, as well as just the basics of how to do things like make the canes smaller ("reduce" them), slice them evenly, etc.:

http://glassattic.com/polymer/canes--instructions.htm
...If you want to make canes of *other* fruits, you can either use the same kinds of kaleidoscopic symmetry techniques as used for most citrus/etc. canes (click on the TRIANGULAR, KALEIDOSCOPE category near the middle of the list on that page)... or you can make canes with images of the *exterior* of the fruits/foods/anything by checking out the "PICTURE" canes (near the bottom of the list), though many of those aren't as easy for beginners.

http://glassattic.com/polymer/canes--reducing.htm
http://glassattic.com/polymer/faces_caned.htm
and maybe:
http://glassattic.com/polymer/canes-whatarethey.htm

Re using polymer clay canes on fingernails though, do keep in mind that polymer clay is oil-based not water-based, so it can eat into some other kinds of plastic over time (if the clay is raw, or if it hasn't been totally-thoughly cured). So anything *directly* touching polymer clay (like fingernails, false fingernails, or clear finishes) should not be petroleum-scolvent based (i.e., clean up with paint thinner) or be certain kinds of stiff plastic.
The clear fingernail polish intended for use on fake fingernails is fine as are other water-based things such as polyurethanes, acrylic mediums, etc.
Fake fingernails themselves are plastic so it might be good to paint them first with that specific fingernail polish to seal them from the clay since I'm not sure exactly which type of plastic they are.

Also, if you want to place the cane slices on the fingernails after they're already cured, you might want to bake them on a slightly curved surface so that they'll have a slight curve and fit better (a slightly curved piece of paper or cardstock or even a glass jar, etc....remove after cooling).

Here are some other pages at my site that you might want to check out for doing this kind of polymer clay thing in particular:
...clear finishes (okay ones, and not okay ones)
http://glassattic.com/polymer/finishes.htm
........you won't be completely "covering" anything with clay, but the principles would still apply for the various materials that come in contact with polymer clay
http://glassattic.com/polymer/covering.htm
http://glassattic.com/polymer/storage.htm

HTH,

Diane B.