1. make a folder for your addon like: Myaddon
path:
steamname/garrysmod/garrysmod/addons
2. copy the Info.txt that is in the other addon folders and paste it into yours Myaddon.
path example:
steamname/garrysmod/garrysmod/addons/phx3/info.txt
3. Rename everything that is inside the info.txt to your own.
4. make the paths, so inside the Myaddon folder make the subfolders:
steamname/garrysmod/garrysmod/addons/Myaddon/maps/mymap.bsp
or / and:
steamname/garrysmod/garrysmod/addons/Myaddon/materials/mytexturefolder/minetexture.vtf
5. if it not works repeat!
6. if it still not works add me on steam: dannneman
Yeah i think its all, bam me if i should make things more clear!