Recently I have seen many people training their triceps and all making a similar common mistake, lifting the weight but not paying attention to form. When working the triceps on most occasions (with an exception of a few exercises) your elbows want to stay tight to your body and keeping your elbows in and not flared out. When you keep your elbows close to your side it will solely work your triceps and not include other muscles like the back. Very important if you want those arms to grow, don't forget you have three muscles in your triceps so make sure you hit every head.
A small tip is to use the mirrors around you in the gym. The mirrors are there for you to assess your technique and make sure you are doing things like this and staying strict with your form.