Official Addons are installed automatically. The user need to redownload the DLC, start it and follow the instructions of the installer. After - run the game, go to the menu item "downloadable content", login with an EA account or BIOWARE (this is a necessary condition) and, if necessary, enter sent along with DLC codes.
If the downloaded mod has format .dazip, then the installation happens using "Daupdater", which can be found in the root directory of the game. Running the software, click on "Select DAZips", and then select all the downloaded files that you wish to install. Then hit "Install Selected": Firefox will be installed in the folder "My documents" → "BioWare" → "DragonAge". Then, open the "Settings" folder in this directory and edit the file AddIns.xml: find all line "RequiresAuthorization="1"" and change the right side to "0". Start the game and in the already mentioned "Downloadable content", select the "Installed content" and check boxes for all installed add-ons.
If you download the Supplement, which contains updated textures then install them manually. To do this, open the game directory and go to "packages" → "core" → "override" and put all downloaded. Don't forget to do a backup when you have to replace some file if the modification will be broken, you can return everything as it was. Possible and more sophisticated methods of installation: if the archive with the mod there is a readme file, then check whether there are special instructions for specific modifications.
You can simplify your life using DAModder and DAO-Modmanager (in essence, they are no different from each other). Are programs that are similar to Daupdater, but much more convenient: they can be used to establish not only modifications, but also texture. You don't have to work directly with files will be limited only by pressing the "Set" menu.