Hey, The best way to enter is from Jaigoan, it'd India Bhutan border.
You can take flight or train till Bagdogra or NJP then take cab to Jaigoan. Transit is easy Passport is not mandatory. You can take flight directly to Paro, Bhutan as well. Check for Druk airline connectivity from your city on their website. Also their is a 1200 rupees Sustainable Development fee charged by government for each day you stay their. That you would need to pay the office where you get your permit. A guide is mandatory post Covid ,that will cost you extra.