The representation of mechanisms does not require certain hoops to be jumped. The problem with a mechanism is that you have to represent as static drawings something that is a dynamic process. An examiner can get a feel for whether the candidate understands the mechanism, and therefore deserves the credit for this, without getting a ruler out to see if the arrows are of exactly the right length. However, the candidate has a duty to make intentions clear by drawing as carefully as possible. Some pointers will be found in each of the mechanism pages.
Curly arrows show the movement of electron pairs .
The tail of the arrow is where the electrons come from, and the head can either point at the attacked atom, or can finish at the midpoint of where the new bond will be. Draw the mechanisms so that as far as is possible electrons move, but atoms do not do so very much. In mechanisms involving nucleophiles show the lone pairs. This is not required , but is very helpful. Don't draw arrows to or from charges.