all: Prebooking vs on the spot room hunting
When coming to Nepal, it is both practical and easy to have pre-booked accommodation. If a bit upmarket, the hotel staff will meet you at the airport or bus station as you are released into Kathmandu, and drive you to their guest house or hotel.
You may make a reservation and provide advance payment or be part of a package deal, or pay upon arrival. My experience is that as the day nears the end, and you arrive late, your room is nevertheless taken, the staff believing - "guessing" - you're not coming. So a reservation may be fictious...
Room hunting on arrival is easy, but you ought to know what you want and approximately what you will spend. Already at the airport you can get a room at the tourism association's desk at the customs exit or from a taxi driver or from a hotel representative frantically waving a signboard. You can also take a taxi to a place known to you (like Kathmandu Guest House) and ask for a room for the first night, and look around the next day for something better/cheaper or in a different location. Around the Kathmandu Guest House there are numerous hotels and guest houses. If you have already booked a trekking tour, ask them to get a guest house for you, too - easy.
Generally there is a glut in the room market - there is always room somewhere. The prices are low, but beware of government taxes and service charges slapped on top of accommodation and food.
Many long-timers in Nepal end up with a preferred hotel to the extent that loyalty supercedes common sense. It is worthwhile taking recommendations with a pinch of salt. That includes my own advice - he, he.... It is the old debate of identity vs market, the one that good old Marx got stranded on in his argumentation on India.
But it is fantastic to arrive in a hotel or guest house where you are made welcome as a family member and upon departure are asked as you please... If you can reference a name or a contact who advised you to a specific hotel, the identity factor is soon creeping into your life, too!