Although a visa is not required to enter Bhutan for Indian citizens, they need a permit, which is free of cost. It is recommended to get the permit in Kolkata itself. You can get the permit from Royal Consulate of Bhutan in Kolkata. Note that it takes 8 days to get your permit issued. Alternatively, you can get your permit issued from Phuentsoling itself. Phuentsoling is a small Bhutanese town at the India-Bhutan border. The problem is that the permit-issuing office remains closed on Sundays and public holidays.
And regarding trek DAY 1- ARRIVAL IN PARO, BHUTAN DAY 2- SOI YAKSA DAY 1 (START OF TREK)
DAY 3- SOI YAKSA DAY 2
DAY 4- SOI YAKSA DAY 3
DAY 5- SOI YAKSA DAY 4
DAY 6- SOI YAKSA DAY 5 (END OF TREK)
DAY 7- DEPARTURE DAY