In this guide I will go through the basic process by placing some guns and stuff, in this case on the CS:S map de_dust. Placing those scripts on a server will apply the new spawns when the map is loaded.
The system works by creating what are called “re-arm scripts” (because you are placing new weapons, hence the term re-arming) using the TTT Weapon Placer tool in Gmod’s Sandbox mode. This is primarily useful for taking a map not created for the gamemode and making it playable in TTT. TTT has a system for creating or modifying the weapon and player spawns of a map.