For information on preparing for this trek, including when to go, what to bring, what permits are required, and safety precautions including altitude sickness, and water contamination, see Trekking in Nepal.
Nayapul is marked by a collection of road side shacks that function as bus/taxi stop backed by a slightly more substantial village that has a jeep track through the middle of it.
Start your trek by following the jeep track down until you spot a swingbridge to your left, there is a foot path leading to it and you cross the river here. Turn left as you come of the bridge and pick up the track along the Modi Khola for about half an hour before you come to the outskirts of Birethanti and a ACAP check point where you have to get your Conservation permit checked. You can now see another swingbridge that you have to cross. Directly opposite the swingbridge is the TIMS office where you can either buy TIMS certificate or just check it if you have it already.
Birethanti has a number of lodges although if you have just started your hike you may wish to walk on to Tikhedunga.
Keep the river to your left hand side as you follow the start of a jeep track, and the destruction of the trail, for an hour or so before it peters out at the small hamlet of Baajgara where there are two tea shops. The trail keeps following the river, make sure the river is always to your left, and eventually you arrive in Hile where there are four lodges. The lodge owners here like to make out you are already in Tikhedunga in order to catch some of their customers. In fact Tikhedunga and it's myriad of lodges is another fifteen minutes down the track.