Depending on the period, shields were plain wood or leather or fabric, hide glued over a wooden surface. Sometimes tourney shields had built up surfaces using gessso. While the little bit of rolled over leather or fabric can look good in some cases, it is often overdone or done wrong, in that there is more leather or fabric "rolled over" than would have come away from the cut surface.
In other instances the leather or fabric or wood (yes, I have seen wood protrayed as this) is puckered outward. And again...why when the cut should either split the surface or indent it rather than make it pucker outward?
The easiest method in my opinion is to just go ahead and make the scoring with a scoring tool or just a plain knife. If you want a larger mark, say from a heavier blade or axe, make the incision v-shape in depth to make it look wider. Then as Keith suggested, make your highlight and shadow lines along it to accentuate the incision. Small gouges or indentation can be done the same way.
Smaller shields like bucklers will have more damage on the edges and the center boss since they are more proactively used to parry and block sword blows.
Think of the weapon that is being defended against and it will give you a clue to the amount of damage that will be inflicted but also remember to keep in mind the scenario. For example, a knight in tournament using bastion clubs will not have sword marks on his shield because the bastion clubs are wood or leather and the idea is to count the number of blows struck or to beat your opponent into submission. Tournaments were social activities by the rich or well off and it was not only how well you fought or rode but how you looked, thus shields constructed for that day would not have the damage that say a shield used by a foot soldier on campaign might use. If the tournament is using rebated weapons "a plaisance" (for pleasure or sport) then the damage will be there but not to the extent inflicted by edged weapons for tourneys fought "a outrance" (for war, to kill the opponent).
Axes leave huge gouges on shields and maces make dents, unless they are bladed or vaned and then there are gouges or punctures from the triangular edges. Continuity is everything.
Mike