New way
=================
1. Crop your image to 256x256 and preferably in PNG-24 if you have transparency

2. Visit https://rafradek.github.io/Mishcatt

3. Click Choose Files and select your image

4. In Dimensions, select [256] x [256]

5. Make sure Sampling Format is set to DTX1

6. Rename to anything easy to type and recognizable like nsf_weed so it exports as nsf_weed.vtf

7. Navigate to your NEOTOKYO installation folder by 
7.1. Right-click the NEOTOKYO game in your Steam Library
7.2. Mouse-over Manage and select Browse local files

8. Navigate further to \neotokyosource\materials\vgui\logos

9. Drag your VTF file into the logos folder

10. Boot the game and go to Settings > Multiplayer and Import Spray...

11. Select your nsf_weed.vtf you just created and moved in \neotokyosource\materials\vgui\logos

12. Open the Console with the ~ (tilde) Key

13. Type cl_logofile "materials/vgui/logos/nsf_weed.vtf" to properly apply the changes

14. Hit the Spray key in game and enjoy your funny spray


Old way
=================

Start up Neotokyo and try to import a spray as you would with any other game. 
After trying to import, the dialog box should close seeming like it worked, but in reality it doesn’t. 
Afterwards you can close the game. 
 
What happens here is that the game creates a vmt (texture detail) file, but doesn’t actually create the vtf (texture) file. 
It also sets cl_logofile to an empty file. Seems like the developers didn’t want to include sprays in the game, but didn’t actually remove support. Instead they just make it tedious. 
 
Then you will have to install VTFEdit from: 
https://web.archive.org/web/20191003040631/http://nemesis.thewavelength.net/index.php?c=238#p238 
You can read more information about VTFEdit from: 
https://developer.valvesoftware.com/wiki/VTFEdit 
 
After installing, open VTFEdit and import the same image you tried importing earlier and scale it to the dimensions 256×256. 
The reason for using the dimensions 256×256 is if the custom spray is larger than 128KB it will not be allowed to be used (server complains that the file is too large). 
I don’t know how to increase the file size. 
 
Once done importing, save it as a vtf file. 
Once you have made sure the file is less than 128KB, copy the vtf file to: 
C:\Program Files (x86)\Steam\steamapps\common\NEOTOKYO\NeotokyoSource\materials\vgui\logos 
 
And then in console run: 
cl_logofile “materials/vgui/logos/myspray.vtf” 
 
Where myspray.vtf should be the filename you used.