Here is a small tutorial to help you start designing your own scenery, the steps should be simple enough with some guide lines.
Download your SRTM data that you require and unzip into a directory of your choice, to help with selecting your scenery area I would download and install Google Earth.
SRTM Data : ftp://e0srp01u.ecs.nasa.gov/srtm/version2/
Google Earth : http://earth.google.com
Download and install 3dem.
3dem : www.visualizationsoftware.com/3dem.html
Start 3dem and max out the window.
Select File->Load Terrain Model from the menu, you should now see 8 different types of data that can be selected.
Select SRTM Data (hgt, bil) format, then click OK.
Locate your hgt tiles that you have downloaded and unzipped, the best option when selecting your tiles is to keep the CTRL button pressed down while selecting your tiles, once you are happy with your selection click OK.
I would suggest to start with only opening a 3x3 grid of tiles.
N01W001, N01W002, N01W003, N02W001, N02W002, N02W003, N03W001, N03W002, N03W003.
After a short while you should see your data being displayed.
The next step is to remove any voids left in the SRTM data, select from the menu Operations->Patch Missing Data or press F7, you should see some text towards the top of the screen, place your mouse pointer just outside of the selected group of tiles that you can see, next press the left mouse button, now as you move the mouse you should see red square or rectangle as the mouse is moved, place the mouse at the bottom right and left click again then press the enter key.
After a short while another small window should indicate how may points have been removed, click OK.
The next step is to convert to UTM Projection, select from the menu Operations->Change Projection->Convert to UTM Projection, you should now see 4 options:
Select WGS84, click OK.
The next step is to save your work, select the following from the menu,
File->Save Terrain Matrix
There should now be 3 options:
Binary - Floating Point
Binary - Signed Integer
ASCII - Text String
Select Binary - Signed Integer
Save into a directory of your choice with a suitable name of your scenery.
When you save your terrain 2 files are saved.
You will now need to convert the bin file into raw format, select the Command Prompt from the Windows Start menu, then locate the directory where you have saved your Terrain files, next type the following to covert from bin format to raw format.
rename scenery_name.bin scenery_name.raw
Press return and close the Command Prompt window.
This is all that can be done for the time being, but should be enough to get you going.