In simple terms, Supremacy revolves around units and provinces. Provinces produce resources, which are used to create units, who then go on to fight for more provinces. Control territory, build up your forces, and take more territory.
Easy, right?
Each province will produce specific resources, the amounts of which will be impacted by factors such as morale or the resource-generating buildings constructed on that province. There is no way to force a province to generate a resource it does not already have, and so if you need a specific resource, you’ll need to expand your territory and take it from somebody else.
Invasions take effort, and effort means recruitment. Units vary, and have different strengths and weaknesses. They also require different resources and buildings to produce, so make sure you inspect your planned army’s requirements carefully before you put your plans into action.
As your borders expand, so will your defensive needs, but as long as you remember to keep the resources flowing, you are on track to victory.