Well, He's a fast one to post

So, to get you an introduction, to put your models in game has some degrees of difficulty, from very easy, like adding weapons, to a bit frustating like armors. Props and characters are somewhere in between.
In your other topic I asked what modelling program you use, so let's keep that topic on there.
The modding knowlegde you need to add models to the game is fairly basic.
For example, for adding a weapons, you are going to need to create a unit file, that defines it as a weapon, this file is going to contain all the data of this weapon, like type (sword, hammer, etc.), level, effects, etc. one of this properties is the one that interest you, and that is resource directory and mesh file. Creating this file is not hard, you can use the GUTS units editor to clone an existing weapon and then just change the mesh file. I have done this a lot for my Ripper's armory mod.
This Tutorial gives a pretty good idea of how to add weapons, was made for Torchlight 1, but the same principles still apply.
Now for characters, it became a little more tricky, editing the unit file of a monster or character class and changing his resourse and mesh files is not enough for it to work. You also need to define his "animation file" this file contains data of what his animations do in the game, for example in the "Attack.skeleton" animation in what frame does the attack hit, and the sounds it plays.
Right now the only progam able to export animations to the torchlight format, that I know, is 3ds max... I don't if maya can do it, since I haven't worked with it. And Blender has some nasty bugs, but I'm still experimenting with it, so don't take my work as absolute.
For some guides on how animations work you can read
this and
thisNow, for your props, there are two ways to add them, as a room piece (purely cosmetic, not interactable) or as a real prop that has interaction. This I'll let Kva talk about since I have little to no experience with how to make them work.