This guide assumes you have Rocksmith installed on your PC. All references to the Rocksmith folder refer to the following default locations.
Steam Version folder location: "C:\Program Files (x86)\steam\steamapps\common\Rocksmith\"
Disk Version folder location: "C:\Program Files\Ubisoft\Rocksmith\"

Setup
  1. Download the latest Rocksmith Custom Song Creator Toolkit.
  2. Extract the contents to a location of your choice. You can put it inside the \Rocksmith\Content folder with no ill effects.
  3. If you haven't purchased, Damn It - Blink 182, you should. Many content creators use this song's App ID in their creations and many custom songs will play without needing to do steps 6-8.

Song Installation
  1. Download a song from the Packages section of the Smithy Anvil forum. You want the *.dat file, which you may need to extract from a ZIP or RAR archive.
  2. Copy the .dat file into the \Rocksmith\Content\ folder.
  3. Launch the Song Creator Toolkit by double clicking RocksmithTookitGUI.exe.
  4. Select the 'DLC Packer/Unpacker' tab.
  5. Select a DLC song you have legitimately purchased in the 'Update - App ID' dropdown menu.
  6. Select 'Choose DLC' and browse to your \Rocksmith\Content\ folder.
  7. To select more than one *.dat file, hold CTRL and click on each one. DO NOT select songs named "RS001SONG0000###". Those are the DLC songs you've purchased through Steam/Rocksmith Store.
  8. Select the Open button and wait for the 'APP ID update is complete.' message. If you are doing a large number of songs in one go it may take some time and there is no progress bar, so be patient.
  9. Launch Rocksmith and test your new tunes.
  10. Repeat from step 4 to add more tracks and voila!

Troubleshooting
If you get an application error after the Ubisoft logo you probably have a corrupted .dat file. Remove any custom songs you imported since the game last ran successfully from the content folder and re-import them one by one, testing the game each time until you isolate the culprit and then re-download that file. It is strongly recommended that you only import 1-4 songs at a time.

Playing 2014 Custom Content