Hi Tom.
The best method is to back-flush the cooling system, so insert a hosepipe into a coolant hose in the reverse direction of the normal coolant flow, turn the heater on hot and disconnect the other main hose.
In theory any deposits that could be blocking the system are dislodged and come out in the reverse flush. You shouldn't run the engine for this task.
You can use an additive to clean it out at the same time but I have never used anything. Be careful to capture any antifreeze to reuse and not allow it to enter the drainage system, etc.
Reconnect the hoses and fill with coolant, run the engine and test for leaks.
It may also be worth replacing the coolant pressure cap as well as this can lead to coolant loss (loss of system pressure)