All you need to know if you start trekking from senaru village
This is by far the most used route as it allows the (relatively) less strenuous trek to the crater rim only. You start at an elevation of 600 m in Senaru village, normally in the late morning or early afternoon and make the 6 hour ascent to the crater rim before dark.
First part of the trek is through tropical rain forest. Look and listen out for the Lutung, or black monkey, swinging through the trees in the late morning. They are extremely shy, unlike for the common macaques which may gather near rest stops waiting for scraps.
The route passes through POSII at 1,500 m, breaches the treeline at 2,000 m (POSIII) and then it is a bit of slog up gravel paths to the camp site just below the crater rim at 2,600 m.
There is no clear path at the last section nearing crater rim, at an incline of 45 degrees. There is a POS Extra after POS 3, but it is pretty much gone, and the remaining bits have been chopped up by porters as firewood.
If that has not sapped all of your energy you can then make a very precarious descent down to shores of the crater lake and bath in the hot waters. You need some confidence to do this as it is a steep descent of 600+ m using handrails and ropes.
And then it is back to Senaru the way you came. Local guides claim going down is faster, but speed varies for individuals. Patience is vital for first timers, as it can get frustrating watching porters sail past you with their load balanced on one shoulder, clad in slippers or even walking barefoot.
Descent from POS Extra to POS 3 can be slippery when weather is dry, as scree tends to give way under your feet. From POS 3 to POS 2, the descent can take 1.5 to 2 hours, as savanna grasslands fades away, and the humidity and heat from the tropical rain forest begins to set in. Useful tip: request for guides to cut out a wooden trekking pole if you did not bring a trekking pole with you.
It will be extremely handy during the descent, especially when legs are aching from the strenuous climb.
There is an option to go on to Plawangan II where you overnight again before making the ascent to the absolute summit. This is not especially recommended from the Senaru side though. If you want to get to the top, the climb to Plawangan II is best made from Sambulan Lawang.