The pinned tutorial is from 2011, and has lost relevance. Wsoliman's tutorial is woefully inadequate. Many denounce GIMP as being simple, and complex to use. I present to you my own, complete tutorial on GIMP and its inner workings. Note that this tutorial is based on the macOS operating system.
Step 1. Download
The official website for GIMP is www.gimp.org. From there, you may download and install it onto your computer. Note that GIMP works for Windows, macOS, and Linux (pick your poison). From there, add it to your list of applications and launch it. You should get a blank screen that looks somewhat like this:
Step 2. Add Templates
You guys all know where Med's templates are. As a refresher, they're here. Download them, and then keep them in a safe and organized place so you know where to access them later (and while you're at it, consider helping Med out with a donation or something?). Once you've done that, double-click on the template to add it into GIMP. The .psd should be recognized by GIMP and opened automatically. If it does not, right-click on it and then click "Open With" and then select "GIMP." As an example, I've picked the 737-800. It should look something like this:
Step 3. Layers
A key aspect of GIMP (and most painting programs) is the layering options. Think of them like an onion, in which one layer goes over another. Note that Med has arranged his templates so that the base layer is in white ("FUSELAGE," for example), and all details (ie. "FUSELAGE EFFECTS") go above it. When painting, make sure that your paint goes in a new layer between the base layer and the detail layers. That way, the paint looks accurate. Also know that some templates, like the 737, offer multiple variants. Clicking on the eyeball will make layers visible or invisible: you can make the regular wing invisible and the wingletted wing visible, for example, to change the variant. That looks something like this:
Step 4. Painting
Now is when you can really get to town. I'm going to use a real-life airline for an example, in this case, something simple like Delta. First and foremost, remove all unnecessary layers. In my case, I've removed the eyebrow windows and the metal overlay.
Now, you get to add a new layer. Directly below all the layers are the layer modification tools: the left-most one will add a new layer. Position it where you'd like, and then name it something memorable. Below, I've created a new layer for the fuselage, and called it "FUSELAGE_PAINT."
Here's the fun part. Select a colour with the two swatches of colour on the left-hand side: change the top one to the colour of your choice. In Delta's case, it would be their signature blue colour, which I've added.
But, how do you paint within the lines? The answer: select tools. The left-hand side is full of tools for this purpose. Most are self-explanatory: the rectangle select tool makes rectangles of the size you drag, the ellipse select tool does basically what the rectangle select tool does, but for ellipses, etcetera. Free select lets you either hand-draw what you want or make a custom polygon out of straight lines. My favourite tool, however, is the paths tool, which is arguably the most complicated. Basically, it works like the free select tool, except you can click-and-drag to add curves to your pathways. When you're done, click "selection from path." I've done this below with the Delta 73H's belly paint.
You may ask now: what about the overhang? This is solved with layer masking, which is straightforward when you figure out what it is. This is going to differ from SKY's tutorial, but the principle is roughly the same. Using the base layer ("FUSELAGE," in this case), go to Layer > Mask > Add Layer Mask > Layer's Alpha Channel. On the paint layer, use the rectangle select tool to crop the layer to be the same size as the fuselage's (Layer > Crop to Selection). From there, repeat what you just did with the base layer, adding a layer mask to the paint too. Both layers should now have a layer mask on them. All you have to do now is hit Cmd-C on the fuselage's layer mask, and Cmd-Z it to the paint's. From there, hit the anchor below the list of layers to adhere the new layer mask. Voilà, conforming paint.
All you really need to do is keep doing this for all the other layers (tail, engines, etc.) until you get a plane that resembles your aircraft. You can also use other tools to add touches you want (ie. the text tool for registrations or aircraft titles), or use File > Open as Layers in order to add logos or whatnot to your livery.
Step 5. Finishing Touches
In order to add realism, consider adding details to make your livery realistic. You can do this by blocking windows, changing door outline colours, etc. in order to get an aircraft that looks like it could work in the real world. I've finished the Delta 737-800 below, complete with registration.
Step 6. Exporting
To export a GIMP image, just use Cmd-E and hit enter. It'll create an exported .png image directly next to the .xcf file in which you've saved the image. From there, all you need to do is upload it and add it here.
And voilà! You have successfully created a livery! Most stuff on GIMP is relatively straightforward, but if you need any help (gradients, colour exchange, etc.), feel free to ask me. I highly recommend trying to replicate a real-world livery, getting to know how to use GIMP successfully before you go on to try to make your own unique livery.