Build some backstory to the proposal.
Your couple is getting married, and in many novels, it wouldn't make sense for them to marry a person they'd never even met before with no explanation as to why. Make sure you've written in the backstory to this proposal before writing it, as it will make the proposal make a lot more sense.
Not every wedding is because the couple is in love. Some weddings are arranged marriages, while others are for "the greater good"
Even if your story has an aspect that makes it so that strangers get married - for example, if a person in the novel's universe grows older than a certain age, they're automatically married to someone regardless of whether they know them or not - make sure that the readers are aware of this. If it's put in the book with no warning, the reader will just be confused.
Set up the scene.
A proposal in a fast-food joint might be humorous, but it's not very classy. The scene doesn't have to be romantic, especially if it's a forced marriage, but it should at least make sense for the situation. And if the character is nervous, show it! As shown in the examples below, it's important to write in your character's emotions. Most stories don't have romances between emotionless robots.
Write the proposal scene well.
Don't make the scene boring, such as the character saying, "Will you marry me?" and the other one automatically accepting. Put in a twist to the writing, and detail the scene well.
Describe the reaction to the proposal.
Not all marriage proposals end in a happy, tearful "yes" - some end much differently. Feel free to experiment around, but try to make sure it fits how the character would most likely react.