The next thing you'll need to do is download this file, D3DX9_42.dll and pop it into your Rocksmith steam folder. This can be found at the Steam SteamApps common Rocksmith2014 directory. Just pop it in, and you're almost ready to start playing custom songs in Rocksmith 2014.
Keep this folder open in your file explorer, though. We'll be coming back to it. The last step here is to open up Steam, navigate to the Rocksmith 2014 DLC page, and buy Cherub Rock by the Smashing Pumpkins. Yes, I'm serious. No, I'm not a Smashing Pumpkins fanboy.
The reason you need to do this is that Rocksmith 2014 custom DLC works by essentially disguising itself as another song. For most of the custom DLC out there, this song is Cherub Rock. So if you don't own it, the game won't be able to recognize any of the songs you download. After you purchase the song, refresh your file explorer window, and you should see a nice little folder marked 'DLC'. Open it up and you'll be able to see all the downloadable content you've purchased for the game thus far. This is where you're going to be saving your custom Rocksmith 2014 songs!
Plug in and rock out! At this point, your copy of Rocksmith 2014 is fully primed to recognize and play custom songs, so all you have to do now is find some! After logging in to CustomsForge, you'll be able to access, a handy search tool for Rocksmith CDLC that allows you to browse songs by artist, album, tuning, and more. Feel free to download as much as you want, but be aware that this is user-generated content so the quality will vary. Usually, the songs with more downloads are of a higher caliber than those with less, so if you're looking for a particularly obscure song, realize that the quality might not be as high as you've come to expect with official Rocksmith 2014 songs. In addition, you're going to want to pay attention to the 'Parts' and 'DD' headings in Ignition. Not all songs offer multiple tracks for lead, rhythm and bass guitar, so be sure your chosen instrument is supported before downloading.