With the points total agreed, players need to pick their forces using the army list in the relevant Warhammer Armies book, and the system presented here.
Unless both players agree to show their army lists to each other before the battle starts, things like Magic Items, hidden units and the like are not disclosed until they are used or appear on the battlefield. Other things that would appear visible on the model such as equipment or specific unit upgrades must be disclosed upon deployment.
Always make sure to make it clear to your opponent what unit is which, and what equipment they have if necessary. Note down what additional Magic Items and equipment each unit might have, along with chosen spells other upgrades and where any hidden units might be deployed so this is not forgotten during the battle.