Hi,
Ok, here goes. As simple as it gets. I'll use this stone block with characters so that faces are easier to identify.
First make a cube and unwrap it. I made a couple of seams and used the standard unwrap function. It gives a basic UV map like in the image.
We can just use this uv map and make a texture for it. As can be seen though, this type of map is quite ineffective. There's a lot of empty space in the texture, enough to fit in another similar cube and more. So let's work on it a bit.
To make better use of texture space, we can scale up the UV map. I've scaled it so that one face fills the whole texture; all the space is now used. The image tiles to fill in the faces that go outside the image area. In our case all the cube's faces are square, so they align pretty nicely here. However, the old texture doesn't really fit with our model, so let's make a new one.
This texture shows only one face, but that's all we need, as it tiles to all the faces. Also notice the increase in detail, as we can now use the whole image to draw one face. The drawback is that all the faces look the same. If the texture and model are well done, that usually doesn't matter.
But what if the model is not a cube, and the UVs don't get aligned that nicely? Then we move them. Select a single UV face and use grab to move it around. One good way to do this is by turning "active face select" on, and disabling "Stick local UVs to Mesh Vertex". Both options found in the UV/Image editor header menus. You can then freely select any face and grab it around. I also had "draw shadow mesh" enabled.
Just grab any faces you want, and move them to the place on the texture that you want. You can use either the model or the shadow image of the other faces as your guide. You can also rotate, mirror, scale, weld, ect. the UVs.
In this case we wanted to overlap the faces, so I've simply moved them all, one by one, over each other. I also mirrored and rotated some, so that they'd look as similar as possible. This was actually wasted work, since Blender has "Reset" Unwrap option, which automatically unwraps each face to show the full texture, but at least we got some experience in moving the faces. Notice that it doesn't matter what size or form the faces are, you can move them just the same way. Moving multiple faces at once is no problem either, just select them all with shift, linked select or similar and grab 'em gone.
Here are the two cubes again, both with 128x128 texture. You can see which uses it's texture space more efficiently. And that's about all there is to it.
Greets,
- Cherppow
