Checking in: you should arrive at airport at least 2 hours before departure time, as the procedures can be lengthy. Driving to the airport normally takes about 1 hour from downtown, but at peak hour it can take up to 1h30, especialy during the rainy season.
You could get an airport bus or taxi from Honoi to Noi Bai Airport:
Taxi price from Hanoi City center (Hoan Kiem Lake) to Noi Bai Airport : Around 170.000 VND to 200.000 VND.
You could also get a particular airport bus: buy ticket at No 2 Quang Trung with the price: 35.000 VND/pax. OR Kumho Viet Thanh bus from 1C Tong Dan, one hour per turn (available from 7 am to 6 pm), Ticket price: 30.000 VND.