I use washes and glazes
Sometimes...Coat D'Arms
ink washes - not the new acrylic ones, which I don't like. I just add tap water.
GW new washes. These are not ink. I think they are very expensive for the thimbleful of liquid you get in the container that won't stack, but they work either straight out of the pot or with just tap water added.
Mixing my own with acrylic paint. Depends on the paint. I have found Vallejo Game Colour works well. Seems that often if the paint is very thin and watery straight out of the bottle it makes a good wash. Again, I just add tap water. I have used washing up liquid and "Kleer" before thay changed the formula, but can't say it was that much better than just using tap water.
The ratios? Sorry, no forumla. Depends on the paint and, I assume, tap water will vary too. If I'm mixing my own, I mix as I go rather than making up a container. But then I only use small amounts.
Tips? Yes. Let it dry. No, really. It needs to dry. If I'm painting metallics - the area where I use washes most - I use my trick of having another figure on the go. So, I wash the metallics - which I would have finished at the end of my last painting session - at the start of a painting session, then paint something on my other figure. I'm quite a slow painter any way, so by the time I have done something on that, I know the wash is completely dry and I can see the result.
Hope that helps.