To get to Shikkotsu Forest, one must first learn the Summoning Jutsu. Then, by reversing it, one can reverse-summon oneself back to Shikkotsu Forest.
"The Summoning Jutsu is essentially a space-time ninjutsu, but it's not hard to learn; its difficulty is only C-rank. The key is finding a summon that you can sign a contract with, a step we can skip. You just need to sign the contract and then perform the hand seals in order," Tsunade explained casually. "It took me less than ten minutes to learn it back then. It's very simple."
"Really?" Nawaki asked, looking at her skeptically.
"Of course, it's true. When have I ever lied to you?"
"Although it's simple, a versatile summon can be a great help in battle, so most powerful ninja have mastered the Summoning Jutsu. You need to learn it well, Nawaki."
With that, Tsunade ran back into the house and retrieved the scroll for the contract with the Great Slug Sage, Katsuyu.
"Remember, there is actually only one slug in Shikkotsu Forest: the Great Slug Sage, Katsuyu. Her body is incredibly massive; every time I summon her, I can only bring forth a small part of her. So, when you successfully summon her, don't say anything foolish."
"Mm-hm," Nawaki nodded repeatedly, impressed by this new knowledge.
He then took the summoning scroll from Tsunade and slowly unrolled it, discovering quite a few names written on it.
"Those are all the people who have signed a contract with Lady Katsuyu in the past."
"Hey, great-grandfather's name is here!" Nawaki unrolled the scroll to a blank section and saw that the most recent name written was Tsunade's, and before that, his own grandfather's—the First Hokage.
"Yes, that's right. Great-grandfather also had a contract."
"Then did great-grandfather know Sage Art?"
"Mmm, logically speaking, he should have."
After observing for a moment, Nawaki asked another strange question. "It seems like great-grand-uncle's name isn't on here. Did he not sign a contract with Lady Katsuyu?"
"If there's no name, it means there's no contract. Perhaps great-grand-uncle had other opportunities, other summons."
"Oh."
Nawaki nodded in understanding. At Tsunade's urging, he signed his name in a blank space and pressed his bloody handprint onto it.
"Alright, now I'll teach you the hand seals for the Summoning Jutsu. Watch closely."
"Mm-hm!"
"Boar - Dog - Bird - Monkey - Ram!" Tsunade released the hand seals in order, then bit her thumb, smeared the blood on her palm, and shouted, "Summoning Jutsu!"
Poof! A cloud of white smoke appeared. When the smoke cleared, a slug the size of a small dog was on the ground. Its body was tender and white with blue markings on its back. Two tentacles extended from its head, and its mouth looked like a chrysanthemum.
Lady Katsuyu slowly raised her head, glanced at Tsunade, and said in a gentle tone, "Oh my, it's little Tsunade. What can I do for you?"
"Greetings, Lady Katsuyu." Tsunade bowed politely, then pulled Nawaki over. "This is my younger brother, Nawaki. I'd like for him to sign a summoning contract with you as well."
"Greetings, Lady Katsuyu. I'm Nawaki."
"What a lovely child. Of course, no problem," Lady Katsuyu replied with a smile. "Go ahead and perform the Summoning Jutsu. I will do my best to respond to you."
"Oh, thank you, Lady Katsuyu."
Following Tsunade's encouraging gaze, Nawaki took a deep breath, began forming the hand seals he had just memorized, and smeared his blood on his palm. He slammed his hand down. "Summoning Jutsu!"
Chakra surged from his body, and another cloud of white smoke appeared, but it was much smaller than Tsunade's. When the smoke dissipated, a tiny, finger-sized slug appeared before them.
"Er..." Nawaki scratched his head in embarrassment. "Does that count as a success?"
"Of course not!" Tsunade said helplessly. "What can something so small possibly do?"
"Don't be nervous, little Nawaki. You'll get it with a few more tries," Lady Katsuyu's voice was as gentle as ever, reigniting the fighting spirit in Nawaki's heart.
"Alright, one more time!"
After a few more attempts, he finally summoned a slug the size of an adult cat.
"Mmm, very good."
The summoned portion of Lady Katsuyu hopped onto Nawaki's shoulder, wiggled a bit, and then let out a soft "hmm?" of surprise. "Little Nawaki, you have a peculiar chakra inside you. This feeling... could it be the Nine-Tails? But why is there so little of it?"
"Lady Katsuyu, you're amazing. You can sense the general situation even through the seal." Tsunade gave her a thumbs-up and then told her everything that had happened to Nawaki.
"I see. So you want to go to Shikkotsu Forest to learn Sage Art and become stronger." Lady Katsuyu said slowly, "But learning Sage Art in Shikkotsu Forest is much more difficult than the others. In thousands of years, only little Hashirama has ever learned it."
"No matter how difficult it is, we want to try!" Tsunade said seriously.
"Me too! If my great-grandfather could do it, I want to do it too!" Nawaki added.
"Mmm, you truly are little Hashirama's descendants. Your personalities are so similar." Lady Katsuyu nodded. "Alright, I understand. I will teach you Sage Art. But you must remember my words, otherwise, you could easily die."
"Yes!"
...
After Lady Katsuyu's patient instruction, Tsunade and Nawaki mastered the knack of the Reverse Summoning Jutsu and successfully arrived in Shikkotsu Forest.
However, the moment their vision cleared, the exposed skin all over their bodies felt a burning pain, even their eyes. They quickly squeezed their eyes shut in alarm. "What's going on?"
"This is an acidic mist," Lady Katsuyu's gentle voice rang out. "Shikkotsu Forest is filled with this corrosive acidic mist. But this level should be bearable for you, right?"
"..." Tsunade used her Creation Rebirth technique to counter it, while Nawaki had already activated his Nine-Tails mode to enhance his healing ability. But even so, the burning pain persisted.
"Don't worry, I can heal you as well."
Two small slugs crawled onto each of their shoulders and began to heal their wounds, relieving a significant portion of their burden.
Tsunade finally breathed a sigh of relief and asked, puzzled, "Why does Shikkotsu Forest have this acidic mist?"
"Mmm, it's actually because of me," Lady Katsuyu said. "I occasionally secrete some corrosive fluid, and when that fluid evaporates into the air, it forms the acidic mist. Thousands of years ago, the air here was actually quite nice. It's all my fault."
"...It's... it's okay. We can handle it."
"That's good. Then let's start learning Sage Art. First, you must learn to sense natural energy."
"Sense natural energy? What is natural energy?" Nawaki asked.
"Natural energy is the energy that roams freely in the atmosphere and the earth."
"Then how should we sense it?"
"It's simple. Soak yourselves in my corrosive fluid. It will help you sense the presence of natural energy much faster. Of course, there's a slower way, which is to just stay in Shikkotsu Forest and use the acidic mist."
"..." Tsunade and Nawaki quickly reached a consensus. "We'll try with the acidic mist first."
"Mm, that works too. The other method is far too dangerous for you. Back in the day, even little Hashirama couldn't last long in there."
"..."