The way I do it is with a machine I have but since you don't have one I would suggest you run a good cleaner in the system first. Run it to operating and for at least 15 minutes after the T-stat opens. Drain the system when it's hotter (no pressure and be careful) Remove the engine block drain plugs (sometimes you need to stick a pocket screwdriver in the hole due to scale build up to allow it to drain. Flush out the heater core and passages with a garden hose. Once you refill the system add a conditioner. It helps prevent oxidation, scale and cavitation.
* fixed typo
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