It's pretty easy actually. You log into your itunesConnect account, click on manage apps. Then choose Manage Game Center.
<img src="http://www.elf-games.com/1.png" border="0" />
Go through all the process of setting up your leaderboard and remember the leaderboard ID.
Then in Construct 2, add the cocoonJS object. On the start of layout request login to game center.
Then using the cocoonJS object choose when to submit a score or access the leaderboard based on the ID you created when setting it up.
Sorry for the quick tutorial, I'll try to do an actual video tutorial of this soon.