It’s always hard to guess what people mean by “simple,” but here’s an idea for incorporating iron, oil, and grain.
() Natural Resources are not accumulated or saved – instead, they are available on each player’s turn (or not). You may use resources that belong to one of your teammates or that belong to an unoccupied neutral country that favors your faction. For example, the United States can use Brazilian iron unless Brazil is occupied by the Axis, because Brazil is pro-Allied.
() Generally, supply must be traced to a unique source. This means that there must be a continuous chain of unconquered territories and/or non-hostile sea zones that connect the factory where you want to build a unit to the resource that is assisting with the unit’s construction. For example, to bring Brazilian iron to the Eastern US factory, you would need either a continuous land route (running up through Panama and Mexico), or a continuous sea route (running through the Gulf of Mexico), or a mixture of the two. You may trace supply through true neutral territories or even through neutral territories that are hostile to your cause (e.g. Britain may trace supply through Iraq even though it is pro-Axis), but you cannot trace supply through a territory that has been activated, conquered, or occupied by your enemies. You cannot use the same resource twice on the same nation’s turn, but you may use the same resource again on each nation’s turn. For example, Brazilian iron could be used to build one battleship on the US turn, and then again to build one battleship on the UK turn.
() available in Norway, Finland, Siberia, Urals, Brazil, Quebec, French West Africa, Western Australia, and Manchuria
() for each BB or CV you want to build, you must either trace supply to a unique source of iron, or spend an extra 5 IPCs.
() available in Iraq, Persia, Saudi Arabia, Venezuela, Egypt, Mexico, Central US, Romania, Caucasus, Borneo, Java, Kazakh
() for each tank or aircraft you want to build, you must either trace supply to a unique source of oil, or spend an extra 3 IPCs.
() available in Ukraine, India, West India, Turkey, Eastern US, Central US, Ontario, Belgium/Holland, Normandy, Sicily, Kiangsi, Kiangsu, Hunan, French Indochina, Ethiopia, Celebes, Colombia, Mexico, Peru, and Queensland.
() for each pair of infantry or mechanized infantry you want to build, you must either trace supply to a unique source of grain, or spend an extra 1 IPC. You may build a single infantry without paying the penalty.