Could someone please explain to me how to add content to the Lambda Players folder, for personal use? I don't want to upload anything, but I do want to add some meme sprays for the LPs to use in-game.

The tutorial on the addon page describes a process that apparently no longer exists, where you have to find files inside the GarrysMod\sourceengine file - see below.

"To add sprays, place the following image files to the directory below: .VTFs, .PNGs, and .JPGs (DON'T CONFUSE .JPG with .JPEG! .JPEG WILL NOT WORK!)
DRIVE:\Program Files (x86)\Steam\steamapps\common\GarrysMod\sourceengine\materials\lambdaplayers\sprays"

Relevant files are now found in another location OUTSIDE of the game files, and are packed into .GMA files. These files can be opened with the GMAD.exe file found in GarrysMod\bin, but attempting to repack the folder and its contents back into a GMA file results in an error.