Anything you add to the paint is considered an additive, even water.
Distilled Water - You can use tap water but it is pretty much guaranteed to have contaminants. A gallon is only a few dollars and it usually lasts me about a year. I've been painting a lot more lately so it has been going quick over the last year but it will still be there for a while.
Matte Medium - This is basically paint without pigment and it will do a good many things. The main features that are good for painting miniatures is it increases Transparency and will remove some of the shine if you are using paint with a gloss finish. Transparency will improved the paints blending properties as you apply layers to transition. (here's a link for greater details)
Slo-Dry Blending Medium - Slo-D will give you a longer palette time which is awesome! It will also increase flow and transparency in your paints which is very nice. Helps your paints blend and gives you a nice consistency .. just don't use too much or your paint will take awhile to dry and could be too thin. (here is a link for full details)
Flow Aid - Flow Aid also affects the flow of the paint and is considered a Flow Enhancer. It will help to eliminate brush strokes from the final project. Like Slo Dry don't go overboard or the paints would be too thin and puddle.(here is the link for greater details)
Palette Wetting Spray - Nothing is worse then mixing up a custom color for something small and then having it dry up before you are done. Sure you should record what you are mixing but sometimes it's a work in progress. If you are working on something you don't want to dry out hit the palette with a quick hit of this. (here is a link with more info)
50% Distilled Water
20% Flow Aid
10% Matte Medium (some times I will increase the amount of MM if I think the paint is too thin)
This has worked pretty well for me so far on most paints but you will notice that almost every paint is different. The color will affect the consistency as well as the brand. Each colors pigments are different and that will usually cause each one to behave a little different. If you've been painting awhile you know that yellows can be a huge pain to work with, the paints are usually thinner and more transparent right out of the bottle, pot or tube.The brand of paint will also be a huge factor, each manufacture has it's own formula and they have their own quality levels. The $1 bottle of paint at Michaels isn't going to have the same finish or work ability as they use lower grade ingredients. I know plenty of people that use them and that's fine but it will be tougher to get a nice smooth finish on your model. If you are going for a higher quality model I'd recommend upgrading your paint.
With that being said lets get into mixing the additives with the paint.
Base Coat -
All Paints -> 1:1 (Additive (A) to Paint(P)) if I want it smoother I'll go up to 2:1 (A:P)
When you are paint a base layer you can paint as normal. You should get a smooth even coat, sometimes it may take two coats to get a solid even color but they should be brush stroke free.
Golden -> 3:1 (A:P) and bump up to 4:1 (A:P) if needed
VGC (Vallejo Game Color) -> 4:1 up to 5:1 .. they usually have a ton of pigment.
VMC (Vallejo Model Color) -> 3:1 to 4:1
VAC (Vallejo Air Color) -> 2:1 usually does it but bump it up in small amounts if required.
GW -> 4:1 to 5:1
Iwata Comm Art -> 2:1
Blending is a different story all together. If you are trying to blend something you will barely need any paint on your brush. Dip you brush into the paint and then dab it onto your paper towel. With the paint being much thinner it will still be there and will probably surprise you. Start working in a wide area and gradually move to a narrow focused section where you want the blended in color to be most evident. You will build up layers of the new color making the color transition you are looking for. The exact amount on your brush is something you'll have to play around with.. takes some time. If it's too much it may pool and if it's not enough well.. you won't painting anything haha..
That is just the rough breakdown but the color can really affect the mixture. I always use less of the additive in yellow. Golden paints are in general much better and usually are very consistent so they stay closer to the same ratio for everything. After painting with the additive you'll get more comfortable and start to know what colors require more mixing effort. Play around with it and have some fun!