Welcome to the asyl— err, forum.
Aah, Trespasser modding. What a pain. It's hard to really answer your implied question on how to mod, so I'll just try to point you in the right direction.
First and foremost, something you require is computer skills. Trespasser is not like Source, with all of the easy-and-user-friendly applications to let anyone do it, Trespasser requires skill, and a whole lot of patience. You might try and fail 10 times before you can get something to work (especially the first time).
Thus, it's not so easy to write up a straightforward guide to modding Trespasser.
Most of the software you'll need to begin are included in Trespasser Twilight:
http://www.trescom.org/files/Editing/Tr ... ght_v1.rar (Hopefully you know what to do with RAR files, if not I can explain.)
Trespasser Twilight is a modded game meant for playing and testing your own custom levels.
If you want, you can also check out my site, Steps To Trespassing:
http://stepstotrespassing.yolasite.com/the-guide.php
Sure, it's incomplete, but hopefully it'll help you a bit.
If you run into trouble, trying searching the forum before you ask a question. Believe it or not, others may have run into the same problem as you. And if you really need to ask a question (which I would be happy to answer), just post it in this topic, don't start a new one.
Good luck.