I've been surprised by the number of artists I've met in classes or workshops who have never tubed their own paint, and whose misconceptions on the difficulty of the task have deterred them from ever attempting the process. Tubing paint is really quite easy, and it is a good practice in which to get into: it can be a convenience if you like to pre-mix color strings before commencing a picture, a time saver if you are painting large areas of a flat non-standard color mixture, or a necessity if you accidentally puncture one of your commercially tubed paints, and you don't want to lose all of that expensive pigment due to it hardening within the tube. For these reasons, it is always a good idea to keep a few empty tubes in your studio.
This is basic overview of how I do it:
1. You need to have empty paint tubes. They are available from many different art supply stores, especially those which sell dry pigments.
This is how they look when you purchase them from the store. The tip is capped, and the back is open.
I prefer tubes with a small neck, like that pictured above, left, but they are harder to find. Mine were purchased from a company which tubed and sold their own colors, after they had closed up shop, and were selling off their production materials. Once my stockpile is used up, I'll be hunting for small-necked tubes again.
2. You'll need a target color (you can't get there if you don't know where you're going!), unless you are just re-tubing a commercial pigment.
3. Tools:
A variety of palette and painting knives are essential, and you should probably already have some amongst your current art supplies. It's a good idea to have more than one, so you can use one to scrape your paint off the other while mixing. The narrow-bladed knife at the bottom is excellent for loading paint into the back of the empty tube.
A table-top, hobby vise is good for crimping the tubes when you have finished filling them, but isn't absolutely necessary. This model has a vacuum-like seal for temporarily attaching it to your work surface. Vises like this can be found at most hardware stores, and run about $25 USD.
Other tools which come in handy are tin snips (in case you are only partially filling a tube, there's no sense in folding over all of that excess tube), flat pliers (for crimping the tube if you don't possess a hobby vise), and a small piece of dowel (to use like a rolling pin for flattening out the tube prior to folding and crimping).
4. You'll need your paint!
I'll be mixing a tube of viridian green lightened with flake white in order to make a color of the value 6 on the Munsell scale. The tube on the left is a neutral value 6 I mixed previously, and I will use that as my target value.
Here I've squeezed out my pigments on my glass palette. The top neutral color is my target value, the middle is my viridian straight from the tube, and the bottom is my flake white. The palette is a piece of 1/4" glass with smooth edges which I picked up at the local glass shop. Underneath the glass I have placed a piece of mid-value, warm gray, pastel paper, which through the glass, appears more neutral (glass contains iron impurities which give it a green cast, most notably seen when looking at the edges- the green in the glass balances the warmer tone in the pastel paper making the final color,when looked at through the glass, appear more neutral).
5. Mix
In this image, I've begun my mixing. I'll keep adjusting the proportion of viridian to white and continue attempting to match my pile to the value I established as my goal until it's finally correct. After doing this a while, you'll develop an eye for how large a paint pile you need to fill a tube. On the right is a widget; its razor blade is great for scraping paint off a glass palette, especially when the paint has dried and adhered to the surface of the glass.
6. Fill the Tube
In these pictures, I am using the narrow-bladed painting knife to load paint into the back of the empty tube.
7. Tamp the Tube
You need to get the paint settled to the front of the tube, without air trapped inside. If you leave air pockets in the tube, the paint will begin to oxidize and harden. I just bang the tube against my thigh to settle the paint (Newton's First Law of Motion: Inertia?). You'll want to leave about an inch and a half between the end of the tube and the line where the paint has settled inside the tube.
8. Clean the Tube
Once the tube is filled and tamped, it's a good idea to clean that last 1 1/2" with a rag or paper towel. When you begin flattening the tube, paint left in this area is likely to squeeze out the end and make a mess.
9. Pinch the Tube
Using just your fingers, you can pinch the tube flat. Sometimes at this stage, I use a piece of dowel as a rolling pin to flatten the tube.
10. Crimp the Tail
In this image you can see the beginning of the first fold at the end of the tube. You can begin this fold many different ways using a variety of tools. You can bend the first flap over a palette knife's edge or over a ruler, use your pliers, or bend the tube over the edge of the vise.
11. Seal the Crimp
Once the flap is bent all the way over, seal it tight in your vise, or with a pair of pliers. Repeat by folding another turning of the flap, and compress it in the vise. Continue this process until you've folded over the excess tube.
12. The Completed Tube
On my 
