Pick whichever one of the people in the relationship who, depending on gender and sexual preferences, could in theory become infatuated with you. Once you have selected this person, start spending a lot of time with them/talking to them. It is not necessary for this to be at all romantic. Once the other person finds out about how much time they are spending with you (even if this time spent was completely platonic), you will have planted the seeds of jealousy.