2000 Sierra coolant flush

I have a 2000 GMC Sierra 1500 2WD, with a 4.8 in it. Recently I had no heat in the truck, which was puzzling because about three months ago I changed the thermostat. So I changed the thermostat again and I now have heat, but I would like to do a flush and fill. I work on my own vehicles, but have never done a flush and fill. Any tips or advice would be great. Thanks in advance.

I flushed the system with a kit available at local parts store, and followed the directions. Then after that I removed all the coolant hoses and flushed with water. Also ran water through the heater core lines. Now I finally have heat. I figured I would post back, in case anyone else had this issue.

Usually heat loss is due to a low level of coolant. Check to make sure you have it topped up and your reservoir is full.( to the mark)
Sometimes the thermostat can stick "open" and won't let the engine heat up. Again,no heat.
When you want to flush and fill,those kits are very handy for getting rid of any junk in the coolant system. Then refilling , you need to top up the rad and start the engine. The coolant level will drop and you keep adding until full. Watch for bubbles and wait till the coolant starts to get hot. Check for heat at your vents.
Sometimes you will notice the engine temp is hot but still no heat at the vents. You need to rev the engine a little and this will force the coolant to the thermostat which will then open and start circulating. This is called an air pocket,somewhat common.
Put on your rad cap and again make sure the reservoir is full to the "mark".

Kevin