You can enter Bhutan by taking a flight to the only international airport of Bhutan in Paro. Druk Air and Bhutan Airlines run flights from major Indian cities (Delhi, Kolkata, Bodh Gaya, Guwahati) and other countries like Thailand (BKK), Singapore (SIN), Bangladesh (Dhaka), and Nepal (KTM). Outfitter Nepal flies you from Kathmandu to Paro (airfare included in the package cost). They’ve three other entry ports on the border of Bhutan and India, but it’s tedious.