Nice work oyvdahl. You've created a resource that will be helpful for beginners, to show them that you CAN do a lot with free tools.
I could write pages on PCB layout considerations, but I'll spare you most of the boredom.
Sometimes beginners have trouble with selecting appropriate components. Capacitors are a good example. Often they're just specified as "10uF/25V" or something like that; the beginner needs to know to use a standard electrolytic, or a ceramic capacitor for decoupling, and so on.
You have to assume that anyone designing a circuit that needs components with special characteristics such as low ESR, or a heatsink attached, for exampe, will clearly mark the relevant components, but that's not always the case. Also, the layout person needs to know how to place and route decoupling capacitors so they will be effective; often these capacitors are shown in a clump in a corner of the diagram, or vaguely positioned near the applicable IC.
Some designs require component construction - e.g. inductor winding.
Perhaps you could do a few detailed case studies, starting with a sparse schematic that uses a variety of special cases like these, and explaining component choice and any other decisions that must be made in the process of converting a schematic into a layout.
For double-sided PCBs, with larger designs especially, it can be helpful to use layer biasing - that is, most of the tracks on the underside are horizontal (for example) and most on the top layer are vertical. You may have covered this already; my apologies if so.
Finally, I personally would use tracks thicker than 12 thou, except in dense areas such as buses. Although modern PCB houses can easily make reliable PCBs with tracks of 8 thou and less, these are more vulnerable to damage from scratches or being lifted along with pads during the rework that often happens during experimentation.
For single-sided boards especially, I often use 15 thou tracks on a 25 thou grid, giving 10 thou clearances. If the layout is sparse, and capacitance is not important, you can use thicker tracks; between nearby pads, I often use a track thickness equal to the pad diameter, especially on single-sided boards.
Edit: a "thou" is a thousandth of an inch; many people use "mil" to mean thousandth of an inch (for "milli-inch" I guess) but I avoid "mil" since it's easily confused with millimetre.