First off, a disclaimer.
Before you install the mod, I recommend you make these changes. Just revert them when you're not using it.
If you're playing using the steam version, then switch to ASCII before you install the mod. You do this by going to in-game settings, then video, then select 'use classic ASCII glyphs'.
I also recommend that, if you're not doing this already, go to the in-game settings, then 'game', then set 'portable mode' to 'yes'. This puts files like saves, worldgen, pref files, and so on within the dwarf fortress directory instead of appdata/roaming, which may make it easier to clear out the mod if you need to re-install it. If you prefer portable mode off then skip this step.
You can download the mod through DFFD or Steam Workshop. The Long Night works identically on the steam or freeware version of Dwarf Fortress, so either of these work. It's just a matter of accessibility and preference.
To install on the Steam Workshop, just click subscribe. However, Steam is not very good when it comes to updates for the game, which may cause errors due to the workshop mixing versions together. I can't fix this, so if it happens, manually go into the areas that mods are stored, that being AppData\Roaming\Bay 12 Games\Dwarf Fortress or Steam\steamapps\common\Dwarf Fortress. If you're not sure, check both. In the Steam directory, the mod's number is '2898804695'. Also, if you are using the Steam version, you have to switch from graphical to ASCII mode.
To install manually after downloading it from DFFD, place the folder in Dwarf Fortress\mods. Do NOT put it in 'installed mods'. If the folder doesn't exist, create it. New updates also need to be manually downloaded. Again, it is recommended that you thoroughly scrub old versions.
Within the folder there is a file called 'worldgen data'. Follow the instructions there and take the copied material to the Steam\steamapps\common\Dwarf Fortress\prefs folder or the AppData\Roaming\Bay 12 Games\Dwarf Fortress\prefs folder. Copy the worldgen data into this file. You do NOT need to override any pre-existing worldgen templates.
This file may not exist for you. If so, go to advanced worldgen in the game, and click the 'save' button.
When you load the Long Night mod, only choose from these world presets in Advanced World Generation unless you really know what you are doing. If something goes wrong and you haven't used these to gen your world, I won't be able to help you, because that can cause a lot of problems I can't fix. You don't need to delete these if you want to generate a vanilla world, just don't choose them when generating a non-Long Night world.
These parameters may change, requiring an update, but I will announce in the changelog if that is done. If you do not have a prefs folder, got to advanced world gen and SAVE the parameters. That should generate one. If you're experienced with tweaking advanced worldgen, you're welcome to adjust these parameters to taste. If you aren't but want to be, study here.
Within the mod folder, there is a file called 'colors'. These alter the color scheme of the mod and are an intended part of the experience, so it is recommmended you replace the colors.txt in the Steam\steamapps\common\Dwarf Fortress\data\init folder with this one. If that fails, try looking in AppData\Roaming\Bay 12 Games\Dwarf Fortress\prefs. Remember to make a backup, such as naming the old .txt file to colors_original.txt. When you want to revert to that, just switch them out.
Don't like the colors? Make your own with this website: https://blomquist.xyz/color/
The short version is there isn't any. The long version is there might be some, but you won't be able to access the new content and depending on the update, some things may be broken. If you want to keep playing in an old world, keep an old version of the mod. If you want to update, make sure the save file is among the things you delete.