To get married to a bachelor or bachelorette, you'll have to trigger a series of events that will slowely entice the candidate to grow feelings for you. After you have completed all of a candidate's love events and you have met the remaining marriage requirements (house level 3, big bed, red flower color) you can use the traditional marriage proposal item to request the person's hand in marriage.
Before you can see a candidate's love events you'll have to see the person's friend events. These events could be considered a white love event and a purple love event, but they are viewable by both male main characters and female main characters, whereas the love events can only be viewed by those who are playing as the opposite gender of the marriage candidate. Most marriage candidates have two friend events that you'll need to see before you can start courting love events, with the Japan-only candidates Woofio and Stephanie having only one friend event.
After the friend events are completed you can start to trigger the love events. Marriage candidates have a love event at a light-blue flower color, a dark-blue flower color, and a light-green flower color. These events play in sequential order (e.g., you can't see the light-green love event if you haven't seen the dark-blue love event) for any available marriage candidate.
To see the next set of love events at a yellow flower color, orange flower color, and pink flower color you need to confirm your commitment with the person. Give a love Pendant to the person you are courting to become a couple, which locks you to that person's future love events. After you give the Pendant you can no longer see love events from other candidates, even if you haven't seen their light-blue, dark-blue, or light-green events yet.
Woofio and Stephanie do not have post-Pendant love events. Similarly, Inari doesn't require the going-steady Pendant. Instead, you'll need to have the Tsuyukusa marriage proposal item in your bag to trigger Inari's last two love events.
Remember: You can see 1 event per day (love or friend). If another villager is involved in the event, being ignored for some time ("Hey, I haven't talked to you in a while") can possibly also prevent the event they're included in from triggering.