1. Finish any track.
2A. You go back to your profile, and click on ghosts. Click on the track you just rode, and copy the URL. Paste your URL to any thread of your choice.
2B. If your ghost by any chance disappeared from your ghosts list, go to the track you've ghosted, and enter /r/*username* behind the URL. Copy the URL and share your ghost in any thread of your choice.
. The link will in both cases end up looking something like this:
http://www.freeriderhd.com/t/225795-mario-pixels/r/stig of which the /r/ represents race, and the username tells you who's ghost it is, in this case, it's mine.