This might be confusing at first because I think game don't explain how to backstab. In order to backstab you have to be behind the target and press "E". You can backstab hostile guards in clan hideouts.
If you killed them you can go to dungeons and farm some firecrackers. Then you toss them before monster sees you. When it will go to check the place you tossed the firecracker you backstab it. After 10 backstabs you should get the achievement.