I am a novice but here is what I have found in both my own experience and my exhaustive search across the web. So with that in mind, please disregard my post in the event of any discrepancies compared to the much more experienced master modelers around here.
I use acrylic paints exclusively. As many mentioned it depends on the brand of paint. Some, such as Vallejo, come ready to paint. Others, such as Tamiya and Model Masters may or may not need thinning. I use the brand name thinner that goes with its paint (i.e. Tamiya thinner for Tamiya paints, Model Masters thinner for Model Masters paints, etc.). I have found that, basically, these thinners are isopropyl alcohol (rubbing alcohol) which I suppose you could use as well. I have been admonished for considering using water but really, I don't know.
As for ratio. You typically want something like 50/50. The consistency of milk is what you are looking for. Often, the ratio is a lot less thinner to paint and I think it can even vary with shades. Seems like 'this' shade doesn't require much, if any, thinning compared to 'that' shade. But I also wonder if it has to do with how long the paint was on the shelf.
For me, I spend the few bucks on the appropriate brand thinner and use that for painting. Cleanup is done with rubbing alcohol. Works just as good but might take a hair bit longer and saves a lot of money. I have been using 90% alcohol but imagine lesser concentrations would work.
I know, I know new guy with no experience sure has a lot to say
How to thin acrylic paint for airbrushing. I gotta learn to cut it down some........