Transpose all of the notes in the guitar down an octave. The guitar sounds an octave lower than written, so to play the same identical pitches on mandolin, you have to transpose the notes down.
Check the transposition to see if there are any notes lower than a G below middle C. If there are, you will need to transpose the entire section up an octave to make it playable on the mandolin. The mandolin's lowest note is G below middle C. You don't have to worry about the high range, since the mandolin can play higher than guitar.
Analyze the chords; the mandolin only has four strings, making it possible to play four notes simultaneously, while the classical guitar has six, making it possible to play up to six chord tones. Since you can only play one note per string, you will have to check the chords to see if you need to omit any notes.