There are 42 sections with 3 stars to obtain in each (126 Stars in total). You only have to get 100 stars, so you don't need to go for 3 in all sections. You need about 2.5 stars in average, which means you should get 3 stars every second time, for the rest you are good with 2 stars.
Note: Don't worry if you played through the game and didn't hit the 100. You can replay single sections at any time from the campaign menu.
I recommend playing in co-op mode. The more human players, the better. All your actions count towards a shared star score. Note that ALL PLAYERS have to choose hardcore or insane difficulty, since you will only get stars for the lowest of all chosen difficulties.
If you are playing with AI, I recommend you stay near your AI mates, so they can quickly help you up when you're down. You don't need to help them if they are down since they can't die. They will help each other eventually.
In order to quickly obtain stars, you should mind six things:
1. Declassified Missions will make the game more difficult, but will also boost the star meter, making it increase a lot faster. I recommend you always activate the declassified missions to get the most "points".
2. Headshots give extra rewards. If you have headshot weapons, MAKE HEADSHOTS.
3. Executions will earn you extra rewards, too. If you have a lancer, you should always try to saw down your enemies. Also watch for downed drones and finish them off.
4. Splatter will earn you extra Gib Rewards. If you have neither a headshot weapon nor a lancer, go for shotguns, boomshots, booshkas, mulchers and the like to make your enemies explode.
5. Ribbons earn you extra rewards, but most of the time you will earn them automatically. You can always try to chainsaw three opponents in a row to get the lumberjack ribbon, or make three headshots in a row and so on.
6. Going down (DBNO) will drain your star meter immediately. Make sure to always stay near the other players, so they can help you. If you play co-op, make helping your human mates a priority. Also note that downed AI WON'T drain your meter.