How does one go about doing this without their brain exploding? I'm a thinker, not a coder
Well to open the file in wordpad, you can right-click the .set file and choose "Open With..." Then choose Notepad (or Wordpad) from the list of programs it gives you. I'd recommend checking the box that says "always open with this program" so that in the future you can just click the .set file (this won't mess up Tile System, don't worry.)
Once you've got it open, you'll see a bunch of lines of code. Here's an excerpt from one of the Descent set files:
<TileType type="0">Floor Tiles</TileType>
============= Map Tiles ==============
<Tile name="RtL corridor/outside mud 2x4" sort="10" type="0" maxcount="1" warn="true" flipimage="descent_WART rtl_outside_4x2_mud.png">
The first line, starting with "<TileType" just tells TS which of the dropdown categories to put these tiles in (type="0") and the part between the tags ("Floor Tiles") is the name that will appear for that category.
The "<!--" and "-->" tags are for comments. Anything between a pair of those tags will be ignored by TS.
The rest of the code is an example for one tile.
The string for "name" is what will appear if you hover over a tile while TS is running.
I forget what the "sort" variable is for, sorry. Just leave that one alone unless something explodes.
"Type" matches the type variable in the TileType tag, it will be the same for every tile you want in this category.
"maxcount" is the total number of tiles of this kind available. Once you've dragged this many copies of the tile into the map, a red X will appear over it on the left. If you drag more a red O will appear. It won't stop you from using more tiles than you have, it just warns you of such.
"warn" should always be true, that's related to the red X.
"flipimage" is the name and path for the image you want to appear on the back side of the tile. If you don't want an image on the back side of the tile (ie: it's the same on both sides or else it doesn't have anything useful printed on the back) then you can remove this variable entirely.
In between the tags is the name and path for the image that will appear on the front side of the tile.
The </Tile> tag closes off the code for this tile. After this would come the next tile in the list.
Your .set file needs to be in the same folder as the TileSystem.exe file, thus assuming you want to store your images in a subfolder, you need to include the path from the root (where TileSystem.exe is) to the image folder. That's why the example code has "descent_WART" in front of both images, incidentally. The images are stored in a subfolder called descent_WART.
Edit: There should be a backslash (the one that isn't "/") after descent_WART in each image path. The forum software keeps removing it from the post, probably because these forums use HTML.
Looking at the Descent tileset again, it would appear the author of that set has broken down the code into a variety of .set files. The .set file that's in the root directory just has a series of pointers to all the other files that are stored in descent_WART. Given that, it might be easier for you to just copy all of the DOOM tiles into the respective WART set files and put all your images in the same folder as WART. (By which I mean copy all the DOOM floor tile code out of the doom.set file and paste them into the descent_Floor_Tiles.set file you'll find in the descent_WART folder.) You'll just need to modify the pathname and "type" variable to match the Descent tiles that are already there.
Wiew. After typing all that out I can see why it might look daunting to someone who's not experienced with programming, but it is really pretty easy once you get the hang of it, honest. =P Compared to real programming languages, this set up is practically plain English. =P