Work your way through all the UV map sections making sure the adjacent maps have the same relative scale. Also try to make sure the squares of the checker texture are about the same size over the entire 3D model. However, the top priority is to ensure the adjacent maps are of the same scale to hide the seams as much as possible.
Step 5: Fit all sections into a square area to utilise the available texture resolution
Now that you have all the UV map sections, you need to arrange them to fit into a square shape as tightly as possible with a small amount of space between them (not too small to make it easier when painting the texture map). This is so that the sections are as large as possible so they use more texture area (hence resolution) so you will end up with a more detailed texture appearing on the 3D model. The spaces between the UV map sections are essentially wasted texture space that isn't used so you want to minimise this wastage as much as possible. This is why many people like to stretch the UV map itself into a rectangular or square area (as I mentioned earlier) so that no texture area is wasted, however, I personally don't like the stretching it can cause. When fitting the UV map sections in a tight square shape, you can only move or rotate the pieces (do not scale them one at a time). When doing this I try to keep the UV map sections that are adjacent to one another (like the neck and body) or related sections (like each arm) together. I also try to keep the orientation of each section as intuitive as possible (like the face being vertical and not sideways). This makes it easier when you go to paint the texture maps.
Once you have the UV sections into a square shape, select them all and scale them to fit the UV square background shown in the editor. You will probably need to shift the sections around to fit this exact square better. This square has x and y (u and v actually) dimensions of 0 to 1. This square shape will match the shape of the texture map. The good thing about a UV map is that it is not dependent on resolution, it's a relative scale. This is why the UV dimensions are from 0 to 1 which is essentially a percentage from 0% to 100%. You can see this range displayed in light grey on each background grid axis as 0, 0.1, 0.2, 0.3,... 0.9, 1.
You don't have to use a square texture shape, however it is far more efficient for the computer to process square texture map sizes that are a dimension of a multiple of 2 like 256x256, 512x512, 1024x1024 pixels, etc. You can also use more than one texture file too, like one for the face and another for the body, however this is usually more inefficient and harder to match the scale and colour across texture maps. It is more inefficient because arranging UV sections into multiple squares usually ends up wasting more space because you now have extra boundaries you need to fit them all around.
You have now finished the UV map! You can then create a snapshot of this map to use as a template to see where to paint your texture map. Select Polygons > UV Snapshot... and now go have some fun painting a texture for your 3D model! How do you paint textures? Well, that's another story..