I tend to create my own textures for various reasons (pride and ego notwithstanding). It just makes things easier when you want to modify something, you don't have to clean up someone else's texture or spend a lot of time rearranging parts and blending everything. Essentially, the bulk of my ship textures are based on a few sets of 'blank' aztec patterns. Most details are hand (mouse?) drawn, using an image of the area of the mesh I'm texturing (the saucer or hull, for example) for a point of reference so details don't overlap and so everything will conform to the shapes. Once the shapes are created, I can then fill them in if needed and replace the colors with an aztec pattern, add more details, etc. It's also smart to keep each type of detail for the texture in a separate layer (the windows, the lines, the aztec, the mesh reference, etc.) so that when you modify one part, you don't have to redo the entire thing. Most image programs such as Paint Shop Pro and Photoshop support the layer feature so it's just a matter of familiarizing yourself with the program.
Even though my Sovereigns are a few years outdated, the basic methods I used on their textures still hold up. I don't remember exactly how I did it, but I created a regular aztec pattern first (typically a 512 x 512 bitmap), then varied the brightness/contrast and other settings and reduced the map down to a 2 color black and white image. This gave the result of a bunch of white boxes/pixels on a black background, remarkably similar to those on the Sovereign. Then it's just a matter of replacing the two colors with the ones found on each section of the Sovereign's hull (see example below). I could also take just the pixels and overlay them on top of a regular aztec pattern for an added bit of detail.
To help get a perspective on things, I'll touch a bit on the basic details of the Sovereign (since it's typically perceived as one of the more difficult ships to duplicate). The first thing to notice is the saucer and it's varied detailing. Since the Nemesis version, the aztecs are much more obvious compared to the subtle panels on the First Contact model. The center of the saucer has the 'pixel' panels, along with inner shapes and complex grid lines. The 'flat' part just outside of it has a less cluttered pattern with fewer lines and obviously darker colors. The outer segment lacks the grid lines but returns to the cluttered set of pixels that the inner set has, again with it's own color scheme. The rim of the saucer has sections of panels along with a darker scheme and cluttered pixels. This same routine applies to the bottom of the saucer where each 'step' has it's own subtly different change in colors. The hull follows a similar style, except the panels have more uniform colors and don't vary as much.
Next up is the 'command deck' of the saucer which lacks any aztec pattern whatsoever. Instead, there are panels and colored shapes (note that they don't extend to the edge of the deck, so there's a brief 'line' of hull color in between). There are also various shapes and boxes, most of which have more shapes inside of them that are essentially smaller versions of themselves. One interesting thing to note (in the green markers) is that some of these shapes have 'inlines' that take the same shape of the panels; this is more prevalent on the secondary hull paneling and adds an extra bit of style to the ship while not being overly complicated. In addition, there are 'spider-web' lines in the same shape as the deck triangle that all converge at the front into a single point.
Lastly, there are the remaining shapes (blue marker) - the boxes, the bridge decks, the cove (hull scoop), and various other tidbits across the ship. Most of their panels have the same shapes, it's just that parts of them are removed, like a puzzle that's missing some pieces. Picture the full shape of a panel, then cut away parts of it (in some cases, a majority of it) and there you have the 'inner' panel detail. Also, some of the shapes are recessed, while others are protruding.
Obviously, I didn't go into the technical aspects and label anything after it's 'real' purpose - I'm simply looking at things from an artistic standpoint. Taking the engineering viewpoint comes later, after you've gotten the basic ship created and can then concentrate on any potential design/realism/construction problems you might have missed.