The absolute simplest tool for working with ASCII model files is a plain text editor. As @TheBarbarian says the simplest models to work with are floor decals. Take the following model -
# model: plc_change_me
setsupermodel plc_change_me NULL
node dummy plc_change_me
node trimesh Plane01
ambient 1 1 1
diffuse 1 1 1
specular 0 0 0
-0.5 0.5 0
-0.5 -0.5 0
0.5 0.5 0
0.5 -0.5 0
0 1 0
0 0 0
1 1 0
1 0 0
0 1 2 1 0 1 2 1
3 2 1 1 3 2 1 1
position 0 0 0.02
orientation 0 0 0 0
selfillumcolor 0 0 0
That is for a one metre square floor design.
Before you make use of it you really need to make your texture. So for this little exercise launch your favourite image editing program. I’ll assume that is the gimp which is available for Mac OS. Create a new image 128 pixels square with a white background. Use the brush tool to draw a simple design in your new image (such as a cross). From the menu across the top choose layer/transparency/add alpha channel. Next from the menu across the top select colors/color to alpha. Click OK in the dialogue box that appears. You should now have your design showing on a transparent background.
Now we save it. From the top menu select file/export. This will open a file save dialogue box. So we give it a name and a .tga extension (e.g. “cross.tga”). Choose a location to save it to (on Windows I always use the desktop) and press the export button. This will open a very small dialogue box. In this box there will be a check box that is probably pre-checked for rle-compression. You must make sure that to uncheck that box. Once that is done press the export button in this tiny dialogue box. Your image should now save in the correct format to the location that you chose.
Turning to the model. Copy and paste the model that I have presented above into a new text document. Assuming that you called your texture “cross.tga” save your model as “plc_cross.mdl” in the same location as your texture. Don’t close your text editor yet because we need to edit the text of your model. We are going to do a search replace. Search for “change_me” and replace all occurrences with “cross”. Save the updated model and now you can close your text editor. This leaves just one thing to do in order to get the new model into the game- edit the placeables.2da file.
Probably the easiest way to get the placeables.2da file is to download this collection of all the 1.69 2da files (the download is less than 1 meg). Open the archive and extract the placeables.2da file. Open this 2da file in a plain text editor and go all the way way to the bottom of the file. Now copy this line -
10000 My_Cross **** plc_cross **** **** **** **** **** **** **** **** **** 1
and paste it into the 2da file after all the other entries. Lastly edit the number at the extreme left of this line so that it is actually just one greater than the preceding line in the 2da file. Save the 2da file to the same location as the model and texture.
To test the fruits of your labour, copy these three files into your override folder (sorry, I’m on windows so don’t know where that is on an apple computer). Open the toolset and create a new module with single area with a flat floor (such as castle interior). Now open the the placeables blueprints on the right of the screen (the icon is a wooden table). Choose any placeble and right click on it. This opens a menu. Choose edit copy. This open the item properties window for the copy you are about to edit. If you want, you can change the name and tag while you are here. The third option, Appearance Type, is the one you want . it is a dropdown box. Open it and scroll down until you find “My_Cross”. Select it. You should now be able to find your model of a cross floor design in the custom placeables and be able to use it like any of the built-in models.
Hope that helps (sorry ran out of time, will explain why you need to do certain things later).