When setting up the scenario some games have you pick between a type of force like infantry or armor and the date of the engagement. These settings should be chosen carefully since they will often times limit you to the force pool that you will be able to pick from. Steel Panthers also makes you choose the skill level of the units, based on Green, Average, and Veteran. Your sources should give you a general idea of the opposing forces skill level. There is, according to historian Stephen Ambrose, a rough correlation between skill, morale, and experience.
Green troops are generally less skilled but with higher morale than veteran troops.
While veteran troops are more experienced but are less likely to risk their lives in suicidal charges. If the game doesn't have the exact version of the hardware that you know participated in the engagement you will either have to substitute to the closest available model or as with the case of Steel Panthers the editor allows you to modify a unit's characteristics. This is a very power full tool in that new units can be brought into the game without having to modify any of the actual game files. This approach also assures that all players of the scenario will be able to enjoy the scenario as you intended without having to install some kind of a home brew patch. You will find that all scenario editors have some kind of limitation.