These are the best instructions I've found for installing Forge on a Mac:
http://www.wikihow.com/Install-Minecraft-Forge
Once the new version of Forge is installed, it WILL NOT automatically show up in your Minecraft Launcher. In your Launcher, click on the three horizontal bars to access Launch Options, and then "Add New." Scroll down to the very bottom -- all the versions of Minecraft with Forge are at the very bottom, after all the regular non-Forge versions. Once you have a new profile (I call it "1.11.2 with Forge" or whatever) it will show up in your list at Launch. I just had to figure this all out again this morning to install a new mod for my son, so I figured I'd write it all down while it's fresh!