Writers usually meet in the Writers' Forum. This is where you can go to discuss possible contributions to mainline Wesnoth writing (campaign dialog, unit descriptions, etc.), work on the Wesnoth Manual, collect reference information about the world of Wesnoth, world-build, and do other writerly things. Keep in mind that this involves bringing together information from all over the Wesnothian universe. In other words, this forum does contain spoilers.
If you have an IRC client, you can also join the #wesnoth channel on irc.freenode.net and talk about lore in realtime.
Note: Any text included in The Battle for Wesnoth must be released and available under the Gnu General Public License. Read this forum post by Jetrel and Wesnoth:Copyrights for more information on this.
Warning: These materials may contain spoilers. This is necessary for writers, who may not want or need to play through a campaign to work over the dialog. If you don't want spoilers, you're in the wrong place.
The World of Wesnoth
The People of Wesnoth
- The factions of Wesnoth
- The races of creatures in Wesnoth
- Eleazar on Orcs and Trolls
- A list of hero descriptions
- A detailed family tree of the Garardine Dynasty
- Secret Lore
The Typography Style Guide details the general standards used by text within Wesnoth. There are two areas where this has a bearing: in mainline campaigns, and in the user interface itself. For user-made content, writers have much more freedom, but any content or writing accepted into the game must follow our established style guides.
ESR has already produced some excellent guidelines on writing Wesnoth campaign material. This guide covers storyboarding, plotting and adding flavor, as well as advice on map design, balance, and some of the pitfalls to avoid.
You can't really have a tutorial on how to write. Everyone has a different process. However, here are some pages from the internet that you could glance at before taking up your pen.