Wat is the proper way to flush my transmission fluid out because my radiator went bad and coolant got mixed into it

 

Welcome to HF.

Remove pan, add a drain plug (either welded on bung or drain plug kit), replace filter and pan gasket, reinstall pan, refill, run then drain using new plug and refill etc.... or go to a shop with a flush machine and have them do it.

Here is another thread to look at Transmission maintenance

 

If your transmission has coolant in it, your engine cooling system will most likely have transmission fluid in it. You'll need to flush both. Head over to YouTube and find some videos for flushing a 4L60e transmission. You probably won't find H3 videos, but you'll find Silverado or other GM truck/suv videos. Basically, your transmission is a fluid pump. It involves the trans output line into a 5 gallon bucket to remove bad fluid, while using the input line to pull fresh fluid from a different 5 gallon bucket. I can elaborate if you'd like. Replace your radiator with an H3 radiator from a manual transmission which does not have integrated trans cooler. Get yourself a stand alone trans cooler.