The foam creates the lift
Whisked whites contain many small air bubbles held by unfolded proteins. In the oven, the air and water vapour expand while heat sets the surrounding structure. The dish rises because of this expansion, not because eggs contain a chemical raising agent.
Make the base compatible
The flavoured base should be smooth, strongly seasoned and cool enough not to cook the foam on contact. Loosen it with about a quarter of the whites, then fold in the rest with broad cuts and turns, stopping when only faint streaks remain.
Prepare for immediate service
Butter the dish and coat it as the recipe directs so the mixture can climb. Fill, level and clean the rim, then bake without repeatedly opening the door. A soufflé naturally settles as steam escapes, so serve it promptly rather than treating some collapse as failure.