its very simple; the basics are tf2 hammer and source2 hammer, everything is build in 256 unit blocks in tf2 hammer, in this case it really does not have anything to do with the prop creation, since units in source2 is the same as tf2 hammer, source 2 hammer is free and comes with destinations or dota2, it does not matter which you use,
source2 hammer is a very intuitive mesh builder, very similar to maya etc, but superior in its ability to import vmf, which makes it ideal to create set piece props since it translates 1:1 into old hammer, if you are familiar with maya etc this might not be necessary since you can export ref geometry directly from hammer but I find source2 hammer to be way faster for doing easy things like what I've made, and if you are familiar with old hammer you will get up to speed easily,
when you have created the ref mesh (or final version) you export it as a fbx, since tf2 does not have ssao you probably want to bake it, I use mental ray in maya,
the best way to export props from maya is with the MESA shelf, which is free and super easy to install, some people prefer other applications, whatever 3d application that can import a fbx and export it to smd should work,
-create the geometry in source2
-export it as a fbx (or obj)
-import the file into a application that can export to smd
-fix the uvs
-update the pivot (to make sense, like in the corner of the mesh, make sure its snapped to grid)
-export to smd
I havent started texturing the models, but I will probably use quixel and photoshop, or just photoshop and a wacom, some people prefer other applications, you don't need to bake ao in maya - some prefer xnormal etc, or no ao at all,
all collision is also made in hammer, then you can easily alight it with the grid and keeping the complexity down, while saving yourself the joyful work of clipping a entire level, I was skeptical at first, but it seems to work pretty good,
overall this is a veeeeeery experimental workflow, but so far it seems decent, I get to spend more time in hammer and less time elsewhere, exporting models is boring, I found it good to make 'em in 5-10 batches,...then you never really get tired, but keep the motivation of testing them out in-game,
right now I got around 30 props which have taken me ~8 hours to make, but they are very simple and low poly, still need to fix some vertex faces, make the textures etc,
hope it helps,