Using the bookmarklet
I put an animated demo of how to use the bookmarklet on the YouFeed Tumblr page.
Or, the copy and paste method
If you're watching a video on YouTube, and you decide to subscribe to that user on feedly, first click onto the user's channel page.
Then copy the URL of the channel's page
Paste it into YouFeed then click the red button
The channel will open up in Feedly, which you can then subscribe to by clicking Follow.