Dips for Triceps:
Keep your arms to your sides and your elbows pointing back. Your upper body should be staight. Lower yourself down until your arms are at a 90 degree angle or whatever is comfortable for you. Don't go beyond this point. Your back should be straight and your head should be up. Hold at the bottom of the movement and then drive yourself up to the starting point.
Dips for Chest:
Use the same form as for tricep dips but lean your upper torso forward.
Dips are a great exercise but if you have any past shoulder problems then they should really be avoided.
That would kill my shoulder. If it were injured, I would'nt go for a full stretch in the bottom position of the rep and let the shoulder area lax. I'd keep it tight throughout the movement. Personally, If injured, I'd just avoid it for a while.