For best results, it is best to manicure your nails as long nails can tear the alginate mold.
Having said that, you can artificially thicken long nails so they have much less of a chance of breaking during the de-molding process. Tape or petroleum jelly can be used to make the nails thicker prior to creating the mold. You still need to be extra careful when removing the hand from the mold so that the nails do not tear the inside of the mold. Keep in mind we cannot guarantee that they will turn out perfectly.