I am just getting ready to do this on my 99 F-150. Winter temps FINALLY arrived in Kansas City and I could tell the past 2 mornings were chillier than usual while driving. Temp showed the truck water temp was hot, but the heater hasn't been performing well. The last vehicle I did this for was an 87 B-2000. I used a garden hose to put onto the heater OUTLET on the firewall. The inlet, I connected to another hose and placed in a white plastic trash can.
Back to the hose at the firewall: At the end of the garden hose, I put a shutoff valve and then another short section of hose which was the one connected to the outlet of the heater core (keep in mind you want to run water through to BACKFLUSH the heater core). After I turned the water on and let it run into the heater core, I turned it off at the shutoff valve and loosened the hose downstream to let some air into the hose. I then tightened it again and turned the flow back on. This let some air bubbles help purge the system.
What flowed into the white bucket could have passed for Mississippi River water. It was totally brown. The heater in that Mazda worked just like new after that.