How I used to terrain back when I terrained:
1. I first conceptually lay out a general layout of a map. (For an rp, I would look for AT LEAST 5 different locations that would be important. Like mountains, a lake, a large city, rivers, an open field, canyon system ect. Then place them randomly, or with some purpose, about the map.)
2. What would be special about each of those regions. (For instance, I would select one mountain out of the range, make it very large and have a cliff face overlooking ruins or something. Make each region have something special.)
3. Fit your dream design into something that Starcraft can handle. (Draw a picture of your imaginary place, using graph paper and only moving about in 2x1 diagonals and vertical lines. I do this mentally, but I used to draw stuff out by hand, it helps and shouldn't take too long.)
4. Make the piece you drew. (This may feel like it takes a while to do, but once you know where tiles are, making huge amounts of terrain will take almost no time. I made the entire terrain of Dam Builders in about 12 minutes, and it still seems better than most terrain I see.)
If you get stuck and something doesn't match, you can try varying the piece your placing, the ones it connects to, or simply change the look of the region of terrain. I used to try to avoid using copy/paste brushes as much as possible, because it really helped in making new things, so that every inch of terrain was different. I do not really make maps anymore, so I don't really care if I repeat stuff now.
5. It should look somewhat decent, but some parts may have looked better on paper, so go back and make little trims or extensions here and there. (For instance, in your original concept, you had a very narrow passageway, but on the map the typical size is either too wide or two small, so edit it slightly to make it just right. This is also usually when you aren't quite sure what you want, but later you can get good enough to do this on the fly.)
6. Make sure the vision is constant. (This is something which very few people seem to do after they terrain. I hate seeming an entire cliff face when I move my ground unit to just the right position, but then having it disappear a second later. BE CONSISTENT!!)
7. Add awesome doodads. (Or don't, it's best to only use them for effects which improve the concept you wanted.)
However, in certain cases, it may be alright to not follow these rules. For instance when your concept is to have a really dense jungle, much denser than sc allows. (Were talking MASSES of trees) I focus less on the underlying terrain, having the most important objective be about maintaining vision and making sure the units can be found in the forest. Then I place sprites to my hearts content.
There are always special cases, but the best way to learn terrain is to just play with the palette, it's what everybody has to do to learn it.
None.