People often have trouble placing a texture accuratly on a model. Doing it with planar maps isn't difficult as you would only have to render from an orthogonal viewport and use the picture as reference. But what if the model requires a cylindrical or spherical mapping?
That's where the mesh unwrap utilities come in handy. There are some flavours to choose from, Texporter and TL Unwrap are the ones I know and use (and they're free!).

Texporter is a utility that will render an unwrapped mesh and optionally shade it depending on mesh topology (stretching regions will be colored differently) 

TL Unwrap is a modifier and is really great because it not only renders a wireframe, but also the solid and shaded mesh. It also displays it as unwrapped in the viewport, so it's easy to determine where and how much something will stretch. This is the modifier I'll be using for this tutorial. I more often use Texporter nowadays because it's faster but TL Unwrap does have an additional trait that I'll talk about in the next tutorial.

                   

In this example, I have applied a cylindrical mapping to a pair of sunglasses (here with separate glass and ear rubber objects). This will cause stretching along the top and bottom of it, but for this tutorial it will suffice.

                   

After applying TL Unwrap to it, you will end up with the unwrapped mesh in the viewport. This comes in handy if you change the placement or size of your mapping as you can see how it will influence your mesh.

                   

You can choose your own resolution. I rendered the mesh in 400x100. This is the shaded only version.

                     

And here optionally with it's wireframe. Now save the file to disk and open it in Photoshop or your painter of choise

                   

In Photoshop, you'll create a new layer on top of the template and use the opacity funcion to fade between the two, not unlike drawing on a light-table.

                   

Here I used an opacity of 68 and painted the texture according to the template.

                   

The finished texture. Not very flashy but then again it's just a short tutorial.

                   

And behold, we have a pair of incredibly cool shades...
Anyway, it's really not much to it, but these unwrap tools are very helpful if you don't have a 3D painting program.