I thought flushing oil went out with the ARK certainly I have never used it since the 60’s.
Regular oil changes are all thats required,
But to answer your question way back when flushing oil was used we just drained off the existing oil topped up with flushing oil ran it until the oil was hot, drain fit new filter top up with correct engine oil.
Would be interested to know if any one still does it.
PS I must say if you don’t know what a sump plug is you should not be messing about with the the cars mechanics.