Ok, If I am not mistaken it will be as easy as to

1- Create a tag image (the image of the spray on the floor)
2- Create an icon image (the image of the icon that will be in the spraypaint window)
3- one for chalk too

4- Place those images in "media/textures/Tags" (inside the mod folder) for tag
5- idem "media/textures/Icons" for icon
6- idem "media/textures/Chalk" for chalk

7- go to \media\lua\client\Conf and open shapeConf.lua
8- add the snippet code to any of the array of shapes under the desired symbol category (for example SpecialMeaningSymbol) like this (for a generator symbol):




?

Ok, it didn't work

EDIT: Wait... I had just modified a test shapeConf.lua file xD, not the original one

Yaaayyy. It works... See for yourself:

https://puu.sh/BpT01/da4226688a.png

EDIT2: Now I made one to mark cars (the ones I have a key for): I didn't get The perspective
right though. Note that the symbol is realistic so as to say that you wouldn't need stencils or professional skills to make it):

https://puu.sh/BpVmY/3c2d008c53.png

Now I just can make any symbol easily with the right perspective (Im using an inkscape extension that perfectly "isometrizes" your selection) so homogenous symbols can be made... Here I revamped the wheelcar and added "F" for Food:

See for yourself:

https://puu.sh/BqFH8/6580be9727.png

what letters have you added and I am not savy on what it is you are talking about so would it be possible to get a walk through or a file update that I can add to mine?

Nice just added my own symbols following these instructions, and they work too!