I hereby swear to have read and understood, before doing any of the following...

IN ALL CASES:
Whatever you're doing, chances are the file contains dialogue, so load the latest TR_Mainland.ESP along or the dialogue will lose data and you'll have to start over.
If the file uses assets not already in TR_Data, make sure to load the Tamriel_Data TR Addon. Don't expect the CS to ever warn you about missing anything from these, because you are skipping all errors.

# Merging plugins #
To merge into release files other than TR_Mainland, convert the latest TR_Mainland.ESP into an ESM (Wrye Mash or tes3cmd can do this). Load it along with the release file ESP and the interior ESP so that you can use the "Combine Loaded Plugins" feature in the CS without including Mainland. Once done merging, paste back comments on the new combined release file ESP and remove the dependence to the Mainland ESM in Wrye Mash. Do the same for TR_Data_Addon.esp if you need to load its assets.

# Merging interiors #
If the interior was from the new forum, edit its claim in the claims browser.
If it was an old forum claim, update this document and add a link to its claim page in the release file page's description as done in this one.

# Merging quests / dialogue #
For any common topics (latest rumors, Greetings...), make sure the newly added dialogue entries appear where they should, not at the top or bottom of the list. If the quest/dialogue file edits any "PLACEHOLDER" entries, clean them out before merging.

# Adding dialogue #
When directly adding on the file, add dialogue between existing entries, not at the top or bottom of common topics or Greetings. When you do this, the dialogue entries above and below the insertion point WILL be dirtied (marked as modified) whether you notice it or not. If these entries are not originally from the release file you are editing (= any dialogue entries that do not begin with * when the file is loaded as active: those from vanilla or from Mainland/other TR files) you will need to clean them. Prefer adding new dialogue by creating a new ESP, which is easier to clean and then merge into the file.

# Editing dialogue #
DO NOT modify dialogue entries called "BEGIN... PLACEHOLDER" or "END... PLACEHOLDER". That includes repositioning them, and also includes removing them and creating "exact" copies elsewhere. The data that matters is an ID that you can't edit in the CS.

# Deleting dialogue #
If in TR_Mainland, don't, it can crash savegames and wrye mash doesn't fix it.

# Editing topic names #
If the only modification is changing letters between uppercase and lowercase, doing it in the CS will delete the topic when saving. Use tes3cmd.

# Editing cell names #
# Editing NPC IDs #
# Editing object IDs #
If in TR_Mainland, do not change IDs. No frivolous edits to cell names either, but if necessary add them to the translation.txt file for the Tamriel_Data patcher. (the current version can't include names with colons)
If in another file, make sure no claims based on the release file use the cell name or ID. If you can't, don't edit.
If editing exterior cell names, update the .CSV file that comes with the section to reflect this.

# Editing scripts #
Do not remove local variables from the script. Put any additional local variables after existing ones, not before or inbetween. This is for compatibility with other scripts that write to those variables, and with savegames to avoid crashes.

# Deleting interiors #
Make sure no claims based on the release file use the interior. If you can't, don't remove.
Update this document and make sure it is no longer marked as merged.

# Deleting scripts #
Make sure no claims based on the release file use the script. If you can't, don't remove.
For scripts in Mainland, unless years old and completely obsolete, it may be preferable to first only disable or remove the content of the script but leave the script with its local variables with legacy comments, for the same save compatibility reasons.

IN ALL CASES:
- don't forget to upload the .CSV file if one came with the section
- Log your changes and POST THE CHANGELOG
You don't need to detail every small change, but do detail categories.
"Made edits around the Molagreahd region" is not okay. "Added rocks and edited gate" is okay.
Unless you are fixing a bug or merging an asset that already mentions this data, include any IDs or names affected by the edit so that the log can be searchable.