Step 1. Base it off your life experiences, television show, or books. Watch/read romance books or tv shows or base your characters on your experience on meeting your significant other. Gain inspiration.
Step 2. Make it realistic as if your main characters are the everyday people you witness or known yourself. The couple you plan on creating are your parents, you, your bff, idk. Like they're real people. They exist on this planet right now and just need author to tell their story.
Step 3. What is the plot, how do they did meet, what similarities do they have, what are their cultural backgrounds, and yadda yadda.
Step 4. Imagination.
I hope this helps because I'm not sure if I answered question and just made it up on the spot. Try researching Vietnamese culture.