Making new models is pretty advanced stuff. Most modelling programs have a steep learning curve and there is tons of tutorials if you search online.
I don't believe we have any tutorial for it because of that.
I would recommend Gmax, which is a free version of 3ds Max, as it allows export to TPM. Blender is something free a lot of people use but it is really tricky to get those models into Trespasser.
I personally use Sketchup Make (2017 free version, last non-online version), and export those models into 3ds Max for texturing and optimization. Sketchup is really simple to use but you still need to texture the model in Gmax or 3ds Max, and that is a learning curve as well. I only learned that myself this year.
