The miqat is the boundary that must not be crossed without ihram when travelling to Makkah for Umrah. On a flight to Jeddah, the closest point on the route is usually Al-Juhfa (Rabigh) or its parallel. Prepare before the flight (full ablution, nails, hair removal, perfume on the body for men before the intention, ihram garments). When the crew announces the miqat, form the intention and start the talbiya. As a precaution, many guides have you form the intention slightly before the announcement.
Schools differMost scholars recommend saying “Labbayka ʿumratan”; the intention of the heart remains what matters. Entering ihram at the departure airport (as a precaution) or only when approaching the miqat are both accepted practices: follow your guide.