Disassemble the song to identify the various parts. Most rock songs have a structure that can be easily followed and consists of a few main parts. To write a good rock melody, concentrate on creating a hook in the chorus, but formulate a complementary verse and bridge melody. Separating the parts will make the process easier to accomplish.
Start with the verse. The verse can have a minor hook, but it is the part where the story is told in the song. Verses are often wordy and contain the bulk of the lyrics. The verse may be simple, or it may have a melody that is played on one or more of the instruments. The vocal melody should complement the existing melody, or create a melody on verse music that does not already contain a melody. Try different approaches to see which will fit best with the music.
Spend the bulk of your vocal melody writing time on the chorus. The chorus contains the main hook of the song and should contain the most memorable lyrics or melody. Try to repeat some of the elements during the chorus. One phrase that is catchy should be repeated with added tag lines to keep it interesting. Rock choruses have big-sounding chords, so create a chorus melody using catchy lyrics that complement the chord changes.
Work on the middle section or bridge. Many rock songs have a bridge or middle section that varies greatly from the rest of the song. The bridge often will consist of a short set of measures that builds up or adds some dynamic flavor to the song. If the part builds to a crescendo, also build your melodies and vocal lines up to add even more dynamics to the part. If the part is mellow, write a melody line that works in that context; try not to become disengaged from the song. If the part has aggression or anger, mimic that by avoiding a melody and singing in a percussive or aggressive style.
Work on the "outro" (the opposite of the intro) melody for the song. Rock songs often end with a double chorus or a new part that is solely there to end the song. Use the chorus melody, but add some dynamics to punctuate the ending bars of the music. Bring the melody an octave higher for added tension or simply sing the same melody as you have been in the previous choruses.