First of all, you both need to have a PSN account. Using Share Play your friend can either watch you play, play themselves, or play "local" multiplayer as if they are sitting beside you.
You go to the "party" option on the PS4 home screen and invite your friend to a "party." Then you load the game you are going to play and press the share button (top left of controller), then choose "Share Play." Select the friend you want to play with and wait for them to accept your invitation. Once they accept, the PS4s will be linked and you can Share Play for 60 minutes.
Once you are in the game, you can hit the share button again then select the option to let your friend take over control of your game. A Playstation Plus membership is required to hand off control, but it is not required if they just want to watch you and chat. I'm pretty sure your friend doesn't need Plus membership, just the person who is sharing.