Once you’re through to Nepal via the Sunauli Border, you can easily catch one of the many buses that are heading to where you want to go.
There will be buses headed out to Kathmandu, Pokhara, Chitwan and Lumbini, just ask on the Nepali side how to get there. Do not book bus tickets on the Indian side, these will most likely be fake.
As I was on the direct bus from Varanasi to Kathmandu, when we got through the Sunauli border (by a hair as we were running late!) we just headed straight out to Kathmandu. That took us an extra 18 hours to reach there. It was a hell of a journey. Next time I would head to Chitwan as it’s not so far.
If you’re heading back into India via the Sunauli Border, you will need to get a local bus to Gorakhpur and then from Gorakhpur to Varanasi.
We were waiting/hoping for an air conditioned bus into Varanasi and we were told it was going to arrive at any minute. But, we waited two hours then just decided to hop on a local bus. The journey from the Sunauli border to Varanasi took around 12 hours.
It was horrendous, the bus driver had a gun pulled on him and there was a fight on the bus, but we made it back safe!
This rust bucket took us from Gorakpur 10 hours to Varanasi, I’m surprised it made it