You can actually make an appointment online. I suggest you to check out the waiting time and make an appointment before you travel to be in safe site. The link to fill the form and to gather all the details you need for visa stamping in nepal is
http://www.ustraveldocs.com/np/np-niv-appointmentschedule.asp . Just navigate through and you should be able to gather all the details.
For the wait time and availability of schedule, it seems like you have to register. Give it a shot. For India , it was around 70 days few months back , so better to be safe than sorry right.
direct link for avaialblity /waiting seems to be this one,
https://cgifederal.secure.force.com/?language=English&country=Nepal