You will likely need to be around the area of Sandakan, if not Sepilok, for the start of your trip. The format for most is to pick you up in a minibus and take you to a river. You will then head down the river (which is infested with crocodiles) to a very, very basic jungle camp. Mine wasn’t flashy, but I enjoyed that. This consisted of huts on stilts with mattresses on the floor and mosquito nets around them. Monkeys would jump from one hut to the others, so food and valuables had to be stored away. The tour company would then take you on excursions during the day and at night to see and hear the animals. I did find there were mainly couples on my trip; however, with not a huge backpacker vibe when I was there, so maybe try and convince people you meet at Sepilok hostels to come with you if you want some company. Awesome experience and highly recommend it! The guides were also great. I also saw a video someone showed me at my hostel of how they saw elephants crossing the river when they were on the boat in the jungle, but that is incredibly rare, so don’t get your hopes up. Also fun fact, the elephants on Borneo are an endangered, unique subspecies of the Asian elephant.