1/ Go to your profile Settings.
2/ Make sure to tick the 'My relationships' box under 'Show' and under 'Enable' the 'relationship request' box.
3/ Go to the person's profile, that you would like to be in a relationship with and click on 'More option' underneath the avatar and 'show "in a relationship"'.
A message will be sent to the user in question and they'll decide whether to accept your request or not. If they accept, a new 'love' section will be installed on both of your profiles.
Make sure you make the right choice ! You cannot love several people at the same time.