This is what I came up with in a quick test. The tiles are create in Inkscape and then exported as pngs, trimmed in gimp and put together to test.
I am planning on making a video tutorial on this as soon as possible. The idea is to show the basic creation of e.g. a grass tile, a ground and a water tile and the creation of a mixed tile for the border between e.g. grass and ground.
Please let me know if there are additional points I should cover in this tutorial.
Keep in mind that it might result in splitting this tutorial into two or three parts in order to stay within the 10 – 15 minute mark.