Well I dont download from google earth but from here, like in the tutorial lainer posted
http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/usually you have to download several close by the coordinates you want to get the right area
after that load the files in terragen, can load all at once. Now you have to select the area you want, probably not the whole area since its huge. So you can resize the area (zoom in) with f6. You can also resize the selection square by clicking and dragging in the top right corner of it. When you think you got all the area you want you got file->save terragenterrain->selected area.
After this you gotta open it with terragen, resize it to a size you want, modify terrain units (I dont fully understand this part) to 0-10 units, export it with the exact settings in the tutorial lainer posted. You can then import the file in photoshop for further editing, remember the settings you saved it in (16bits Intel, 1 channel).
You will have to save it in the same format for it to work in bf2editor. Also beware that the size of the file has to be around 2mb wich the file you got from terragen is not. So to get this file you will have to create a dummy heightmap in bf2editor first with the same size you want your map to be (like 1024 pixels). That file will always have the right filesize wich means you can copy the picture from your terragen file and put it into the bf2editor file.
In photoshop you do stuff like trim out some unwanted parts, wich means when you resize it back to the correct size some areas will stretch. You might want this actually since normally the terrain is very compressed (the terrain have the same height as irl, but it is much smaller leading to small spiky mountains etc). Also some blurring is a must since everything is usually very blocky if you open in straight in the editor. Basicly you do all kinds of resizing of areas, brighten (heighten) areas etc because the editor is worthless for editing large chunks of terrain.
Note that you may have to change the mode from 16 to 8 bit to edit it fully, remember to switch it back again. Also the editor reads the file upside down, so always rotate the image before saving.
Here's the tutorial about basic photoshop-heightmap
http://forgottenhope.warumdarum.de/tutorials/heightmaps.phpanother thing to remember about realworld data is that if it has water, like sea or a river, the heightmap wont reflect this, so usually the lowest point in the map will be water
surface height. This will mean that when you add water to your map it will be put
on top of the water surface height. Thereforce it is necessary to increase the contrast between water areas and land areas manually, that is water areas will have to be much darker than the land areas.
you can get a quicker 3d "preview" if you load your heightmap in geocontrol, but it also exaggerates the height a bit so you cant really rely on it. Plus you cant tell scale in relation to ingame objects anyway. As for other uses of shaping in geocontrol or terragen like zeno does is not explained so I have no idea how to do this. Like you said the info to each different step in the mapmaking process is scattered all over the internets and its a pain in the ass to piece together everything.