At the age of 10, and already mad about art, I saw a television documentary that traced a journey along the Sepik River through the jungles of Papua New Guinea. I was entranced and knew that, one day, I would have to draw there.

Fifteen years later I was working as an expedition artist for Operation Raleigh, the charity that offers young people adventure, character-building challenges and volunteering opportunities. Our journey took us through South America, New Zealand and the Torres Strait.

I hadn’t forgotten Papua New Guinea. While in Australia, I bought military maps of the Sepik, and was tantalised by the large white areas marked “relief data incomplete”. The temptation could be borne no more. By the time I found myself on Thursday Island, just north of Cape York Peninsula and only 70ml (110km) from Papua New Guinea, there was no doubt where I was heading next.

I flew into a drought-stricken Port Moresby, and managed to hitch a ride on a missionary Cessna aircraft. I was weighed on a set of scales before boarding, and accompanied on the flight by a filing cabinet. It took several attempts before we found a gap in the cloud so that we could land on a tiny grass airstrip in the village of Kokoda.

The filing cabinet and I were left on the airstrip before the aircraft took off. I was overwhelmed by the humidity, the noise of the rainforest and the smell of a festering dead python, 3m (13ft) long, that had been laid out along the airstrip. I have never felt so stranded and alone.

I stayed with the family of a flight attendant whom I met on the trip to Papua New Guinea, in a tiny village of thatched houses on stilts. As dusk fell and the fireflies came out, the villagers found the only chair they possessed, sat me down beside a kerosene lamp and gave me a huge bowl of steaming sweet potatoes. The entire village sat in the dark to watch me eat. I ate until I was almost sick, but I was never going to finish that enormous pile of food. Would I offend? I was horrified when the remaining sweet potatoes were passed around the village children.

I got a place to sleep on a cocoa barge as it travelled up the Sepik. We stopped in villages where I would sit and draw local people. The drawings took time, and people would sit next to me and talk and laugh at every line I drew. But they brought me food and introduced me to the village elders. It was a wonderful way to get to know people and to strike up trust.

One elder with a withered leg and a bone through his nose thought my drawing might take his spirit away. But after a consultation with other elders he returned the following day to sit for me. He marvelled at my inks and brushes, showed me how to chew a stick to make a brush, and how to mix white paint from burnt coconut husks. He turned out to be the village spirit man and he gave me a painted carving that remains one of my most treasured possessions.

After a week of eating crocodile meat, flying fox and a catfish caught using locusts as bait, it was time to head inland and find those white areas on the map that I couldn’t get out of my mind — the unmapped parts of the Sepik.

I walked alone along jungle tracks, and as I left the flood plain and moved into the hills I could hear the thumping of drums made from hollowed-out tree trunks. I later discovered that several villages were heralding my arrival and that they believed me to be a missionary.

When I arrived at some villages, everyone would run away; in others I had to shake hands with all the residents. Elderly women sometimes had only a couple of fingers (mothers chop their fingers off at the knuckle to grieve when a relative dies). Children would run screaming, thinking I was a spirit, but I would invariably come across a family who were happy to adopt me and feed me.

One man sat naked, patiently clutching a human tibula sharpened into a knife, while I drew him. It had magic powers, he said, and would kill his enemies with the slightest scratch.

Inevitably, after two months, there was a price to pay for trying to hack it alone in the jungle. I was painfully underweight, my ankles were covered in ulcers from leech bites and I went down with malaria. An elderly man walked all day and returned that night with a bottle of Coca- Cola. The sugar seemed to relieve the fever for a while. I was humbled by his kindness, butI knew that it was time to move on.

I had climbed the 4,500m (14,800ft) Mount Wilhelm, the highest peak of my life; dived with hammerhead sharks; sketched sunken Second World War aircraft and ships underwater using waterproof paper; and seen a volcano erupt at night. I had walked the Kokoda Trail, where Japanese and Australian troops fought so bitterly in 1942, and found remains of hand grenades and ammunition. I had stayed in villages that weren’t on any map and, above all, had made friends with the most generous people I had ever met.

Today, as a soldier and war artist who has served and drawn in Iraq and Afghanistan, I can see that my trip to Papua New Guinea as a naive 25-year-old was a life-changing experience. I’ve had many adventures and assignments since then, and I thank that uncharted river and those kind people for setting me firmly on my path.

Sketches from Afghanistan by Matthew Cook is available from www.armybenfundshop.org, £9.99 or www.tolbooks.com. Original works are for sale from Wednesday at matthewcookwarartist.com

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