If the tone is airy, press a little bit harder. The bow should be drawn straight down, not back. Finger placement is tricky, because they move all over depending on the key the music is in. They must be placed firmly down on the string, with bent knuckles. The fingers must not be collapsed.
Short nails are necessary.
They are, however, relative to the positions. Where the first and third fingers are in the normal first position determines third position. The most common positions are first, third, fifth, and seventh. Third is when the first finger is slid up to where the third finger had been. Once in this position, fifth position is sliding the first finger up to where the third finger is again.