Set variable to file content

I am trying to set a variable in puppet to the content of a file located on a server. The file will only contain a string. After some research, it seems a custom fact is probably the way to do this, but I cannot grasp the concept entirely. I am somewhat clear on *how* to write it, but I'm not sure *where* to write/store the fact, if that makes sense. Can anyone help clear up if this is in fact the best way, and if so, give a little better walkthrough than the Puppet docs? Thanks in advance.