Do you have the rulebook? If not, it can be downloaded from BGG. Supplying (or not) is then handled automatically by the software. The landing procedure is explained in detail in the rulebook.
1. Prerequisites
Before spending any points, you must verify the geographical and meteorological feasibility:
The Target: The assault must target a coastal hex that contains a port or is adjacent to one (whether friendly or enemy). You cannot land in the middle of a desert or on an isolated island without any nearby port infrastructure to ensure logistical support.
The Weather: The hex targeted by the invasion must not be affected by severe weather.
The Range of Convoys: For Western Allies, the land unit that will land must be in a port located within one sea zone of the landing zone.
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2. Step-by-Step Procedure
The landing takes place during the "Operations Phase," specifically during the Actions sub-phase.
A. Spend the Surprise Attack token: Operational prerequisite.
To clear the way, you must spend a "Surprise Attack" token in the Sea Zone where the assault will take place. Without this token, the landing craft cannot be launched.
B. Activate the unit in the port: Movement Point expenditure.
Activate the allied land unit (usually an army corps) that is currently in its home port. You must pay the required Production Point (PP) cost for its activation.
C. Declare the target hex and resolve the combat: Resolving the assault.
Designate the targeted coastal hex. The assault is resolved immediately using standard land combat rules, but with one major difference: the attacking unit automatically receives the Naval Support DRM die modifier.
D. Outcome and Occupation: Consequences.
If the combat is successful and the defending hex is captured (or if it was initially empty), the allied unit advances and occupies the hex. If the attack fails, the unit remains in its home port or suffers the losses resulting from the combat.
--- Crucial difference from the Axis and Soviets:
The rules grant a logistical advantage to the Western Allies. The Axis and the USSR must be stacked with a convoy unit that has available sorties in the same sea zone to invade. The Western Allies, thanks to their historical naval supremacy, can launch the assault from a port located up to one sea zone away.
Keep in mind that the game on BGA automatically blocks the action (amphibious landing) if the "Surprise Attack" marker has not been placed beforehand in the designated adjacent Sea Zone. Without this action, no landing!