This assumes you are playing on a PC, if you are a console user, I cannot help, sorry.
First you are going to need the DA:I Mod Manager, available here:
http://www.nexusmods...tion/mods/116/?
Follow the instructions for installing the Mod Manager, also create a folder on your computer for hosting your mods, this is refered to in the tutorial as your 'Mod Manager Folder'. Drop any mods in here.
The you will need to patch file, available here:
http://www.nexusmods...s/?id=493&pUp=1
For getting it all to work I found this helpful.
http://www.nexusmods...tion/mods/493/?
The instructions there can be a bit ambiguous, so a clarification was posted in the comments section by DMDean831: Here's what he has to say:
"This solution has worked for me each time I f'ed up this is kind of a tedious process.(only if you have origin)
Okay step 1: go into your dai "update" folder and delete EVERYTHING in it!
step 2: Use origin to repair the game
step 3: once the game is repaired, open up mod manager folder, if you have a Patch.daimod DELETE IT, now open up the Mod Manager!
step 4: Merge the mods you want anywhere on your desktop(you wont be needing these)
step 5: once everything is completed, close the mod manager, delete the Patch.daimod it just created after merging in step 4, and replace it with the Patch file downloaded here.
step 6: Now create a folder called "merged" into the Update folder.
step 7: Using DMM merge the mods you want again now into the newly created "merged" folder.
step 8: once completed you will want to rename the folder "Patch" to "PatchV" and then rename your ""merged" folder to "patch"
AND IT SHOULD WORK!!"
This worked for me too.
Basically, you are ensuring that your patch is absolute clean, and unmodded by deleting all the contents of the folder, than using the repair game function in Origin.
Then installing the mod manager,
Then merging once with any mod, and saving the results to your desktop, rather than in DA:I's patch folder.
Then swapping the patch.daimod file that it creates with a new patch.daimod available within the link posted above,
Then doing a second merge, with the mods you want to use.