That's just how to drain the boiler, not the system. If you want to drain the system you will need to find the lowest point on your system and drain it from there but you'll have to open up a bleed valve in the highest point in the system to let air in after you have opened the drain cock. You should have a drain off valve around the lowest point.
It might look something like these:
View attachment 268416 View attachment 268417
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