Load up the Cubase session with the recorded vocal tracks you are trying to mix.
Solo your vocal tracks. If you have vocals on multiple audio tracks, hold down "Shift" and click on the "S" button on each vocal track. Listen to the entire vocal performance carefully, preferably with headphones. Fade out any unwanted pops, clicks or harsh breaths in your vocal tracks. When you're happy with the way the raw vocals sound, un-mute the rest of the song.
Create three new "Group" tracks by clicking on the "Project" menu and then selecting "Add Track," and then "Group Track." Label one group as "LeadVocals," another group as "Ad-libs" and the last group as "Chorus."
Assign your vocal tracks into the right group. To put each vocal track into the appropriate group, click on each individual vocal track then click on the "Out:" drop-down menu and select the group track in which the vocal take belongs. For example, your lead vocals should be assigned to the "LeadVocals" group and so on.
Click on the "LeadVocals" group track then select "Inserts." Click on the "i1" box. A drop-down menu will appear. Select "Dynamics" and then click on "Multiband Compressor." Select "Vocals" from the preset menu. Tweak the frequency bands gently until you are happy with the way the vocals sound. Repeat the same steps for the "Ad-libs" and "Chorus" tracks.
Experiment with different effects on each of the group tracks. You can add up to eight inserts (effects) on each group track and Cubase has plenty of stock plugins. For lead vocals, a slight plate-reverb or simple delay can make the vocals sound more natural. Creatively panning your ad-lib tracks can give the song more depth and space. Choosing a built-in Cubase "Modulation" effect can give your chorus presence and make it stand out.
Adjust the volume of your vocal-group tracks by moving the volume slider. You want to make sure you do this with all the instrument's tracks playing as well to ensure your vocals are clear and cut through the music.
Use your ears, not your eyes. There are no set rules to mixing vocals and experimenting is key in getting the perfect mix. Don't get caught up with all the fancy graphics and plugins in Cubase. Instead, focus on the energy of the track and the emotions that the vocals are trying to convey. What works on one song might not work on the other so be patient and listen carefully.