The tools you'd need to make a mod depend on what you want to mod.
Image Editor
If you want to edit image files, you'll need something capable of editing .tga or .png files (GIMP, Photoshop or even Paint will do).
Text Editor
If you want to edit or add game data, you'll need a plain text editor (like Notepad or Notepad++, but NOT Microsoft Word or other formatted text editors).
StringWrangler
If you want to edit or add game strings (usually necessary when adding new objects to the game, can also be helpful to give your mod a title and description), you'll need StringWrangler, a Longbow-developed tool downloadable from https://www.longbowgames.com/downloads/StringWrangler.zip.
FiatXML
To overwrite the .fi files used in the default sandboxes, you'll need https://pub.longbowgames.com/misc/FiatXML.zip to decompress them into hand-editable formats or compress XML files into .FI files. (be sure to use Tools->Re-Indent when exporting to .xml and the two other tools when exporting to .FI). Note that this is only necessary to change existing sandboxes, as new ones can be left in .xml format.
Longbow Resources
Various resources (xnt-files, example mods, etc.) may be found at Longbow's Google Drive
3D-Maps
A file with the HGT file extension is a Shuttle Radar Topography Mission (SRTM) Data file. HGT files contain digital elevation models, which are 3D pictures of a surface, usually a planet, obtained during the Shuttle Radar Topography Mission (SRTM) by NASA and the National Geospatial-Intelligence Agency (NGA). Used here, "HGT" is just an abbreviation for "height." The file is normally named with the longitude and latitude that the image pertains to, within one degree. For example, the file N33W177.hgt would indicate that it includes data for latitudes 33 to 34 North and longitudes 177 to 178 West.
To get the highest quality files, go to the EarthExplorer -site.
1. You will see a map. Click and drag the mouse to pan the map, use mouse wheel or +/- buttons in the top right to zoom.
2. You need to select the area on this map that you want data from. To do this, click on the map to set waypoints and you will automatically see a red area appear between these waypoints. Do this until the red area covers the entire area that you want to download .hgt data for.
3. Then in the left bar click on "data sets" once the area is selected. In the next screen on the left side select Digital Elevation>SRTM>SRTM 1 Arc-Second Global. Click 'Results'
4. On the next screen you will get a list of the files you need. You need to press the rightmost button by each file to download them. This button will be greyed out unless you create an account and log in on this site.