With regard to comments about teachers giving nail advice, it is far from being an easy thing on which to give prescriptive advice. What works for your teacher will not necessarily work for you, because everybody's fingers and nails are different. A decent teacher will acknowledge this fact, while suggesting certain approaches for experimentation. The provisional nature of advice about nail shape is made clear in the literature of pedagogy and technique, as the following examples show:
The shape of the nail is the most important factor in producing a good sound and will indirectly affect the right-hand position. Keep in mind that everyone has a totally different shape of fingertip, each nail grows differently, and each nail has a different density. As a result – in spite of all the marvellous theories about how nails should be shaped – there is absolutely no way to generalise and say "this is correct, and this isn't." (Anthony Glise, Classical Guitar Pedagogy)
These ideas for shaping your nails are only suggestions. There are many variations on the four nail types discussed in this book. While I have found that the corresponding shapes suggested here work well consistently, experimentation is encouraged. Find out what works and feels best for you. (Scott Tennant, Pumping Nylon)
For example, some method books (e.g. Noad, Duncan) suggest that about a sixteenth of an inch (approx. 1.6mm) of nail should be visible above the fingertip when viewed from the palm side of the hand. But this depends very much on how near to the end of the finger the living part of the nail bed extends. On my own hands I cannot see any nail from the palm side, even though I have 2–3mm of white nail extending beyond the quick on each of my fingers, because I have a relatively short nail bed. If I were to grow my nails to the length suggested, they would be much too long for me.
One thing that you could try is to fold a sheet of micro-mesh over one of the strings of your guitar in the area of the sound hole where you normally play. Then 'play' the string a few times with one of your fingers, so that the abrasive sheet wears down your nail along the line that your nail actually travels on the string when you make a stroke. This should give you a good starting point, but really it is a process of trial and error. Your ears must be the judge of when you have achieved a suitable shape; looking at other players' nails is pretty much useless.