Hi,
I just finished shooting two Tamiya 1/16 Scale World Figure Series, 36306 German Machine Gunner http://www.tamiyausa.com/product/plastic/models/36304.html and 36304 WWII German Infantryman http://www.tamiyausa.com/product/plastic/models/36306.html both with Tamiya Acrylic paints XF-10 Flat Brown and XF-65 Field Grey both were thinned with Tamiya X-20A.
Throughly mix the acrylics, then as Jimbolino said pour (I use a glass eye dropper) the thinner in the colour cup first. This will help in mixing.
Last step I take a tooth pick and mix both paint and thinner.
Do a test on some wood to what ever I have around to see if the thickness is right.
I am a Tamiya only man. I buy non-Tamiya tools. However, everything else from putty to thinner is Tamiya. I have tried all the most of the Testors line and have found that you get what you pay for and I can not paint the Model Master Enamels. No matter how hard I try it always comes out really crappy. Go figure...
Tamiya Vs Testors is like NVIDIA vs ATI, IBM vs Apple, Coke vs Pepsi there will never be a winner, it is all a matter of choice.
dss902