One of the first exercises I tried on arm day was tricep dips, but there are so many ways to do them incorrectly and hurt your shoulders and back! Let's take a look at how I've gotten stronger, improved my form, and have learned to avoid injuring myself.
Before photo (left): I am overextending my shoulders and rounding my back. This is bad posture and can injure the shoulders from this angle! Ouch! At this angle, you're not utilizing the triceps properly to push up. Also I'm using my legs too much here and my body is farther away from the bench. Be sure to keep your body upright (no hunching!) and use the arms.
After photo (right): Much better! My body is more upright, I'm utilizing the proper tricep muscles, and my arms are bending at the proper angle. My head and back are not hunching over like in the before photo (a likely sign of weak triceps and using other muscles to overcompensate). I have better posture when my body is closer to the bench.
If you've never done a tricep dip before, try them today! You can do them with bent or straight legs. Sit on a bench and put your hands under your glutes below your shoulders. Shift forward off the bench and lower your body by bending at your elbows, ensuring the your elbows and wrists stay in line. Then push back up to starting. Be sure not to lock your elbows when pushing back up and try to stay upright and avoid using your legs to push up!
If your muscles are weak like mine were in the beginning, try doing it with proper form, but only dipping as low as you can go. As you get stronger, you can get lower your body farther and farther to the ground (Kayla's guide says to go down to a 90 degree angle with your arms). I've learned that as we get stronger, we need to listen to our bodies and challenge ourselves but not push ourselves to the limit of injury. It's OK if you take baby steps to getting stronger every day!