There's always an offer being made. In improv, you never need to make anything up. All You Do Is Accept Offers.

Someone else puts some reality into the scene. That triggers your imagination to see more reality. You put that into the scene. That's all you do.

For example, your scene partner says, "The redcoats are coming." You don't have to think of anything clever. You don't have to "make" anything happen. Just let yourself see the redcoats, and then speak. Speak whatever that image gives you: "I see the whites of their eyes." It's best if you don't even know in advance what you're going to say.

Everything is an offer

What do you do at the beginning of the scene, before an offer has been made?

Actually, everything around you is an offer. The color of your scene partner's shirt. Your scene partner's height, gender, shoe size. Your scene partner's attitude in that very moment. The hardness of the floor. Whatever blurry vision you see if you look toward the stage lights. I got "The redcoats are coming" by looking nearby and noticing a red bottlecap.

And of course, the audience Suggestion is an offer. Let that trigger your mind, and the audience will know that you're using what they've given you. They will feel like part of the performance.