Mixing ink can be challenging. The most economical (and my opinion fun) way of buying ink is to but all the basic process colors and mix them yourself. The Pantone basic colors are in the pantone book www.pantone.com/pages/pantone/index.aspx and if you don’t have one, you should definately get one. For those who don’t have one, the basic colors are as follows: Yellow, Yellow 012, Orange 012, Warm Red, Red 032, Rubine Red, Rhodamine Red, Purple, Violet, Blue 072, Reflex Blue, Process Blue, Green and Black. You will also need transparent white! These colors, when mixed together properly will make the other hundreds of pantone colors. If you don’t mind paying an extra $30-$40 you can have the ink manufacturer mix the color you need for you.
Those who are going to be mixing the inks themselves, here is what I do. I use a piece of glass (make sure the edges are rounded off) or acrylic about 10 inches x 10 inches and some ink knives. The pantone book will tell you the colors and the percentage of each color that you will need to make your specific pantone color (thats why you need to get yourself a pantone book).
Example: Pantone 2738 U (U = uncoated paper and 2738 = dark purple). It says that you will need 75% Reflex Blue and 25% Violet. Since you are not a machine, you will not be able to scoop out the exact percentage. It is also easier to convert the percentages into ratios so, 3:1. With that you can get a small scale and measure out your 3:1 ratio. If you letterpress just for fun, you can get away with mixing by eye and don’t have to worry about remembering your grade school math (lucky).
Now you know how to mix ink, get to it. Happy mixing.