So my
video tutorials explain the basics here... But beyond that, some basic points.
-A typical small trnobj for placing around a map is a simple square with a single texture on it, fit to the edges of the square
-Easiest way to make a new small trnobj is to export one from a retail level's basement (the area off-map which contains organized base models - press "P" in TresEd to show obj locations as red/green dots), then give it a new texture by naming the new texture to the name of the original or opening the TPM file in Notepad and editing the material text
-When making a new (typically 256x256) terrain texture, just make sure it has the same palette as the other trnobjs; this can be accomplished using "Wally Palette Editor" in "Trespasser Twilight" (see sig link)
-Larger trnobjs, such as the light brown dirt/leaf-looking rounded "cover" trnobj in retail JR, use repeated tiling textures and cover more area with fewer objects
-The largest trnobjs are used for the bottom-level ('base') and might be giant squares, hexagons, specially-designed shapes, etc.
-Pay attention to "SoundMaterial" - a trnobj has a soundmaterial-area equivalent to its maximum rectangular dimensions based on the location of the pivot relative to the mesh; if you're not careful a large, unusual-shaped trnobj may cover an area with its sound where its texture is not present, causing one to scratch one's head for a bit
Hope that helps -