The golden rule for DM-Friendly certification is that a published module can be successfully run "out-of-the-box" by the DM in a multiplayer session. This means that a DM who has no previous familiarity with the module can use the included documentation to run it successfully.
Link to the DM-Friendly module list on Neverwinter Vault
Technically, the module will need to include:
-- For NWN and NWN:EE modules, the DMFI Wands and Widgets package.
-- For NWN2 modules, the DMFI Tools package.
(Note: if a functionally equivalent technical package is included in the mod for in-game DM and player tools, then it would also be considered for certification.)
There is no single format required for documentation and a module author can structure their documentation how they see fit. One helpful article is "Creating DM-Friendly Documentation" by legacy DMFI members Jason Robinson and Arawen Silverstar.
To meet minimum standards, the following three categories of information should be included:
1. Module ruleset and requirements. This covers what death, bleeding, rest and other basic systems are used in the module and how they work in-game. Most systems are straightforward and would require only a short description. If the NWN default systems are used and no new scripting systems have been added, then that can simply be stated. Also included in this category is a description of the intended use of the module - how many PCs and of what level, and if any PC class or race combinations should be specifically included or excluded.
2. Area notes. What is supposed to happen in each area? What encounters, quests, and NPCs are in each and how are they intended to be run by the DM? Are there any special events that can occur, either scripted or run by the DM?
3. Plot background and progression. Is there a specific campaign/world setting? How do the PCs start the module and why? What are the major plot points and characters? How should the plot flow?
The following documentation categories are highly desirable and can be quite helpful for DMs, but are not absolutely necessary:
4. Location in the creator palette of plot and other custom items. (This can be a lifesaver if somehow a key plot item is lost and must be replaced by the DM.)
5. Location in the creator palette of custom NPCs and monsters. (Useful for beefing up monster encounters or reviving a destroyed NPC.)
6. Supplemental materials, for example a separate player's guide, a map, or a background description of the module's campaign setting.
Finally, in order to receive certification, the module will have to have been successfully completed by a DM who belongs to the DMFI and is a member of the Review Team. This is intended to validate the golden rule for certification given above, that the module can be successfully run "out-of-the-box" by a DM, and it is not just about documentation. For example, if a total party kill always occurs in a particular area due to an overpowered encounter, all the fancy documentation in the world won't help the DM.
The DMFI welcomes submission of mods by their authors for feedback and certification, as well as volunteers for the Review Team. Any Review Team member must have completed at least one verified DMing session on Neverwinter Connections, see this linked topic for details on the process.