(optional) edit weapon description in config\text\eng\string_table_enc_weapons.xml.
if you want, you can reskin this weapon - textures are located in textures\wpn\, file name: wpn_weaponname.dds (FT 200M example: wpn_fn2000.dds) look at editing *.dds files below.
edit file using information provided below to modd your weapon.
your weapon is located in config\weapons\, file name: w_weaponname.ltx (example: w_fn2000.ltx for FT 200M).
This is the simplest way, however you can't edit unique weapons: I will give you a short description for each way: There are three ways to mod weapons in stalker.