I’m not sure I understand your question, but here goes…yes, the first note is where you
start, then you sometimes have to figure out the duration of the gliss by counting
backwards from where it ends. Occasionally they will write the first note as the actual
length of the glissando. For example, a dotted quarter note as the first note of the gliss
means that the whole gliss lasts for that dotted quarter note’s duration. And yes, you
follow the direction of the squiggly line up or down. Sometimes conductors will ask you to
add more flourishes, even when the music doesn’t show it. Always note the dynamics of a
gliss! Sometimes they diminuendo instead of crescendo. If you have a hand free, it’s
helpful to grab the last note with the thumb as you gliss up to it with the other hand. That
way, you can look at the conductor and yet not worry about missing that final note.
Sorry…you got more than you asked for!