Both destinations are served by Indira Gandhi Airport (DEL). Since there's no flight route between them, the best way to travel is by road.
Delhi
Fly to Indira Gandhi Airport (DEL), then travel by road to Delhi. Taxis, buses, and private cabs are available from the airport.
Sonipat
Fly to Delhi (DEL), then travel by road to Sonipat. Local transport options include taxis, auto-rickshaws, and state buses.