India (Time to process: two hours)
India is different to any other country in that its customs areas are not closed compounds, but public-access places. Therefore, if you drive through this border, be prepared to have your vehicle inspected not only by the customs personnel, but the whole of the village population, and those from further afield who heard of your arrival in advance on the grapevine.
The border gate is at the end of a straight road. As you don’t enter a customs compound, or pass any barrier, you could find yourself at the main border gate without having been through immigration or customs.
So, keep an eye open for the gate, and when spotted, 100m in advance of it, keep an eye open to the right. There is a building (unmarked) with a large kiosk outside offering “official money changing” facilities. This is the entrance of the immigration building.
Enter at this western door, and go right down the corridor to the last door on the right.
Fill in a form, and then hand your passport and form to a guy wandering atop a 6-foot square ledger, as one would wander around a patio, looking for a dropped button.
Scribble your address and passport details in a few dozen places, and sign randomly around the page as directed by the ledger-wallah.
From here, continue westward to the customs hall containing an airport style x-ray machine. This is mainly for foot passengers with suitcases. Present your Carnet to any official, who will then contact the correct official for your vehicle’s inspection.
Then have your vehicle inspected by 500 locals.
Following the vehicle inspection, the Carnet is completed, and you re-enter the public zone to drive to the border gate. To a guard who has no idea whether you have cleared customs with your vehicle or not. He understands the visa stamps, but is clueless about vehicle Carnets. How you handle this will depend upon how narked you have become with the kilometres of pointess Indian red-tape you've just had to unravel, which still leaves a Bengali-only speaking guard on the border gate who has no paperwork proof of whether you've cleared all customs formalities or not.
Bangladesh (Time to process: to find a candle - 15 minutes, to complete the paperwork - 15 seconds)
Cross the border gate, and immediately stop on the left next to the immigration and customs buildings.
Similar to the Indian customs area, the Bangladeshi customs area is a public-access zone. However, as Bangladeshi staring is not as intrusive as the Indian variety, you are generally left alone.