Step 1: Download the 1.05 "daorigins.exe" file:
https://www.dropbox.com/s/bv2m5nbxfz6pr6y/Clean%20DA-O%201.05%20.exe.zip?dl=1
(Move the "daorigins.exe" file into: C:\Program Files (x86)\Steam\SteamApps\common\Dragon Age Ultimate Edition\bin_ship\)



Step 2: Download the 4gb patch (spoiler: it's not actually 4gb)
http://www.ntcore.com/4gb_patch.php
(Run the "4gb_patch.exe" file and browse for the "daorigins.exe" file you just moved)

This attempts to address ALL dialogue, quest, scripting, plot, and MOST item bugs in the base game (850 bug fixes)
Download QUDAO Fixpack v3.5 from link below:
https://www.nexusmods.com/dragonage/mods/4689/?tab=files


Once extracted the file will be in .dazip format.
Go to game file location: Steam\SteamApps\common\Dragon Age Ultimate Edition\bin_ship\
Find & open the application 'DAUpdater'

Click on 'Select DAZIP's' option
Choose the downloaded QUDAO's .dazip file
The file will appear in DAUpdater, select it again and choose 'Install Selected' below
Wait for the installation.

Improved atmosphere mod link: https://www.nexusmods.com/dragonage/mods/577?tab=files
Download 2 Files from here-
IA 2.0 Beta
IA 2.0 Beta Fixes

Extract both
Open Improved atmosphere folder-> Delete the 'Compatibility!!' folder
Open Improved Atmosphere Fixes folder-> Delete the 'Compatibility' folder

Go to game's location Steam\SteamApps\common\Dragon Age Ultimate Edition\packages\core
In here make a new folder named 'override'
Open the 'override' file-> drag and drop Improved Atmosphere folder in it, then open 'Improved Atmosphere Fixes' folder, drag & drop the contents in it to Improved Atmosphere folder you just copied.
Choose the 'move and replace' option when it's copying.


mods apart from .dazip format, like Improved atmosphere mentioned above & .gda files, these can be directly dragged into your override game folder. Make sure to read description though
Example of other drag and drop mod (Fast combat mod): https://www.nexusmods.com/dragonage/mods/2577?tab=files&file_id=12605