Once you have received your routing clearance and taxi clearance, you proceed to the holding point—the marked position at the edge of the runway. There you stop. You perform all remaining pre-takeoff checks: engine parameters, flight surfaces, trims, fuel, and systems. The tower knows you need time for these checks and does not expect an immediate call. Only when you are truly ready do you make the call.
The "ready for departure" call tells the tower you are prepared to receive further instructions. You announce it by saying either "ready for departure" or "will report ready for departure" (depending on whether you are already ready or will be soon). The controller then knows your status.
Important: Always include your holding point location (e.g., "at holding point Runway 27") when you make this call. This ensures there is no confusion about which aircraft is ready.