How do Iclean and flush the coolant line

Replace the thermostat. When the engine is hot. There isn't a hose on it that you can hold for more than a couple seconds. 200+ degrees.

Yup, flush the coolant ayatemavoid using Barsaleak type leak treatments for leaks. They are notorious for clogging up.heater. cores.
a fresh air actuator door can also get stuck open allowing cold fresh air which in wintertime feels like no hot air at all. I think this door is actuated by vacuum hose.

need to make sure you have enough coolant and if it does then it could be your thermostate or clogged heater core

Thermostat.? if you haven't? Do it first then try it. While the car is running for a while touch the heater hoses that lead to the firewall. if one is cold and one is hot? You have a blocked Heater core. Follow the hoses back to the engine. Unconnect them both. By using a garden hose flush out the heater core do one side and then reverse by doing the other hose Lots of junk will come out so be care ful. Mistake people make is "Flushing the system" That doesn't mean your flushing the heater core.
Seeing that you have done everything else it could be the thermostat which you didn't list or the heater core being clogged. When you finished installing the thermostat and flushing the heater core out. Replenish the coolant system . If you have a bleeder on the thermostat housing? use it . loosen it and top off the fluid until you see fluid comeout of the bleeder. The system being air bound would cause no heat..

It may be a problem with your vent control system in your dash. You have a vacuum supply that comes from the top of your engine and travels through your firewall and under your dash to be used to activate the movement of your vent controls. If you have a break in this vacuum line, your vents will not move to change from cool to hot air regardless of where you put your control dial.
That could be your problem.......... or your heater core could be plugged. The core is a mini rad that is under your dash. It takes in hot engine coolant and acts as the heat source. This mini rad sometimes plugs up from ordinary use. Deposits slowly gather up and eventually clog up this rad making it useless. This rad may need to be replaced. Even though your pipes going under your dash are hot, the flow is probably very very slow and not giving enough flow to create heat for you inside your car.
You maybe able to flush out the mini rad by waiting till the engine is cold, then disconnect both hoses under the hood going under the dash, blow out most of the coolant remaining in the heater core(if you can)( also be careful of your eyes) then block one of the lines and put some rad flush through the other line(use a funnel with a hose attached). Leave it there for a while then try running water through it with a garden hose. Be careful to not splash the rad flush fluid in your eyes(wear safety glasses and also wear gloves. Repeat if you feel. Try to flush out as much of the rad flush as you can before reattaching the hoses. Remember to add some coolant cause you will lose some. It would be better to add coolant to the heater core before you attach the hoses to get most of the air out of the system. Good luck

You need to check the heater core it has two hoses going to it feel of them if one is hot and the other cold heater core is stopped up replace it or flush it out if they are both hot it can be the switch or a vacuum line the vacuum line will be on the passenger side under the hood where the airconditioner hoses go in the body

With the engine warmed up feel the heater lines going into the firewall. You will probably find that one is very hot and the other only look warm or even cold. If this is the case the heater core needs to be flushed. With the engine cold remove the hoses at a convenient location and back flush the heater core. Reconnect the lines and warm the engine back up with the heat on.

Does the heater blow cold air from the vents or do you mean the there is no airflow from the vents? cold air from the dash vents when the heater control valve is set fully hot could be an air pocket in the cooling system. The radiator sits slightly lower than the engine causing air pockets to get trapped in the engine when you replace the coolant. you'll have to purge the cooling system to restore flow to the heater core. No airflow (blower motor inop) this could be a bad blower motor transistor. pin test the blower motor circuit.

try changing the thermostat for starters.

You probably need to bleed air out of the cooling system. There may be little brass bleed screws on key high points under the hood. Start the engine (cold), loosen them, remove your radiator cap, and let the water pump work the air to the vents. While you're at it, turn your heat to the hottest setting to circulate coolant through the heater core and flush the air out of there as well. Add coolant as necessary to top it off. When the engine begins to warm up, the coolant will warm up and begin to "grow". Close vents and put cap on. Make sure your overflow tank is filled to "full hot" so that, as the coolant "shrinks" on cool-down, it will pull in coolant and not air. You may need to bleed air off a few times in the future to get it all out but be careful that you don't do it when it's hot because it's easy to get a steam burn.