Determine the duration and time for your solo. For jazz solos and orchestral cadenzas, this may be equal to the duration of the original piece, or the head of the tune, and played after the original piece or head has been played once. For show tunes or movie scores, this may only be a few measures in the middle of the original piece.
Identify the stated or implied harmony for the duration of your solo. For jazz and orchestral solos, the harmony will be the same as the original piece.
Transpose the harmony up a major sixth. The alto saxophone is a transposing instrument in the key of E flat, so the transposition makes sure that you will sound the same notes as are written for the concert score.
Use the notes of the harmony to create a motif. Stay between the G in the middle of the staff to the G on top of the staff for a mid-range motif that leaves you room for development.
Develop the motif to fill the duration of the solo. Augment the motif to create long, soaring notes that show off the alto saxophone's timbre to its best advantage. Include a high, long note on the tonic or dominant toward the end for a climatic solo.