Download TexMod (seems you already did)
Download a... tpf? file - specific to what map you want help with
Open TexMod
Tell it where you GW.exe is, load the whatever tpf map you want to use
Hit play or launch or whatever
It's been a while since I've used it, so sorry if some things are a little off.
Click the big folder icon in the Target Application section and select Browse. Find your GuildWars folder and select your gw.exe. Switch to "Package Mode". In the "Select package" section press the small folder icon and find the .tpf file you want to use and select it. And then hit run, if all goes well your mod should work. Enjoy.