Chels, have a look at this tutorial Sam did.....

Super Sonic Sculpting

What he's suggesting is to apply a bead & create a white tip area upto but not onto the nail plate, just so it buts up to the free edge (click on the pics and they do come bigger btw). Then ensure you've got your basic length & shape the sidealls. This helps your 2nd bead of white to stop slipping around all over the place, which is a nightmare!
After that, get another bead of white (the 2nd bead) & apply just below where you want the smile line to be and work as normal etc just as for a tip & overlay.
This way, the first layer of white stops at the free edge and your actual top layer with your smile is higher up and hides it.

That's how I interpret the tutorial & it works for me.
Try it that way & see how you get on.
I do a thin layer of clear if i'm doing glitters etc to give me the base.

hths xx

thanks bev.. im not sure why i have missed that tutorial? believe me wheni say i have scoured this joint!! haha..
you are right.. ido find myself slipping around a bit with my first layer.. but with this set i tried (miserably) to extend the nail bed.. something i have never seen done or beenn taught.. so i made it even harder for myself!
thanks for your help!

If you are extending the nail bed and sculpting, I would apply a thin layer of clear and butt upto free edge and get basic shape, just the same as if I was sculpting my 1st white bead.
That's just how I would do it, maybe more experienced sculptors will do it another way? then apply my white to the area I wanted to smile to be, that again, does stop all the slipping around!

I think it's hard to extend over a coloured form, because you can't see where you want to extend to. I think it's easiest to do clear first or use a clear form (which I don't like), or a tip.