A and B used to go out. Write down how long for.
They haven’t seen each other or spoken in 6 months, since they broke up. They run into each other. Write down where they run into each other. This will be the location for your scene.
The primary subtext for A throughout the scene is: I miss you.
The primary subtext for B throughout the scene is: I’ve met someone else and we’re engaged.
For each that is what in some way is pulsing underneath the scene. It’s what’s going through their heads. It’s what they think, feel, and perhaps want to say. Maybe it’s even what they’re trying to say. However, for the purpose of the exercise neither is going to actually say either of those things. You know it, we know it, but it’s never allowed to be said.
Now write the scene.
Reading the scene back (or indeed getting your friends/family/flatmates to read the scene aloud for you), what did you notice? Did we get a sense of some of the subtext throughout the scene without it ever becoming overt text?
More importantly, look at the ways you communicated it without saying it. Was it through choice of words when talking about something else? A pause or a beat? Something the character did? The pace? Unfinished thoughts/sentences? Repetitions?