There are two ways to get into Bhutan – by air and overland.
By Air:
You can fly to Paro from Kathmandu, New Delhi, Singapore and Bangkok. The national flag carrier, Druk Air, and Bhutan airlines are the two airlines that you can use to fly into Bhutan. You need a return ticket to travel to Bhutan. Bhutan flights are sometimes delayed and sometimes take off before scheduled time, thus it is recommended that you check in on time. It is also recommended to keep 24 hours transit time for your connecting flight to minimize the risk given the delays of flights due to weather conditions.
Overland:
Overland entry to Bhutan is possible via India Bhutan border. The border towns of Phuentsholing, Samdrup Jonkhar and Gelephu allow access to Bhutan after having necessary documents for Indian border clearance. Make sure you have all necessary documents for border clearance by contacting embassy. Those borders have certain working hours in a day so you need to make sure that you cross the border within the working hours.