All of this has been good advice. It is not hugely important to drain every last drip of water out of your HW tank. Pull the plug and drain the tank. Use the bypass to shut out the HW tank. Blow or run pink through the system. Replace the plug to avoid any rust forming on the threads on the plug and in the tank. Don't mess with the bypass until you get ALL of the pink stuff out of the lines. Introducing pink into your HW tank will create and issue trying to flush it out. I did this once and it took many, many fill and flush cycles to fix things.
I blow my lines because Airstream designed my water system in such a way it is almost impossible to install a hose to add the pink stuff to the system. Blowing works but you have to be careful and not skip any steps. 9 years now without a huge problem.