I won't lie to you - it's very easy to become a good sniper in this game. Most of the assault rifles have scopes and there's also a different category for sniper rifles. You will receive bonuses mostly for long distance shots. Scoring headshots is equally important, so you should always aim for the head. Shoot enemies in the chest only if you're having some difficulties killing moving targets. You may as well ignore other possible bonuses, because it's more difficult to receive them. Some of the terrorists will use ropes, however you're only going to kill about 20-30 of them throughout the course of an entire game. This can go both ways, so if you're using a rope, consider turning around and surprising them.
This, on the other hand, seems to be the most difficult category for scoring points, especially if you've chosen one of two higher difficulty settings. You can receive points mostly for close quarter combat actions, however you should know that these activities are often very dangerous. As a result, you should look for points elsewhere. I would recommend trying to surprise enemy units by shooting them in the back or by blinding them with a flashbang grenade.
Scoring points here seems to be a little easier than in CQB, especially if you like to use grenades to eliminate enemy troops. I suspect that you'll receive these bonuses quite often. Like I've already said, you should focus on throwing grenades at larger groups of enemy units. As for other bonuses, only two of them are worth mentioning. You will receive points for killing enemies by shooting through cover (mostly wooden structures) or by eliminating shielded opponents, however you can't go for headshots, because that would end in receive a bonus in a different category.