Not really a tutorial but I learned the conventions for mirrors first then translated them into the ones for lenses.
Firstly and most importantly, establish what sides of the mirror/lens are in front and behind.
For mirrors, it's simply identifying whether or not it's in front or behind the mirror. If it's in front, it's positive ( + ). If it's behind, it's negative ( - ). This the rule for focal length, radius of curvature, image distance, and object distance. Easy.
Now once you've got that down, for lenses all you have to do is identify whether its a converging or diverging lens. If it's a converging LENS you use all the sign conventions for a converging MIRROR, *EXCEPT* for the image distance. The same applies to diverging lenses (Again, *EXCEPT* image distance - i.e. image distance is negative if image is in front of lens)
Hope this is helpful.
Steve