The picture in my avatar was taken doing hot tub and snow contrast baths. If you look close you can see steam coming from my body.
I have used hot and colds from everything to helping me sleep to preparing for competition to loosening up tight or injured muscles.
We have always done 3 minutes hot followed by 1 minute cold and repeat this 2 more cycles ( 12 minutes total). Often I extend the hot and or cold but initially ( day one , day two) I would begin with the normal ratio.
We have very hot water and almost all year around very cold water.
Ideally dunk tanks are great but if you have sufficient water pressure the shower can be enough.
IMO I also think you get max benefit from doing them consistently over time.
I have discovered the less I want to do a hot and cold the more I seem to benefit from it.