Shouko would tell her past self that it's okay to take chances, to put yourself out there and try. Sometimes trying is more important than anything that follows. Also to check out Ryoka way earlier so they'd have time together before the war started. Also to pick up archery sooner, she has a real knack for it. The future is unclear but Shouko is optimistic about who she will become.
Tsume would tell herself to slow down, just a little. Every day is a miracle but sometimes she moved too fast to really enjoy that. Small steps are what matters, not how fast you make those steps. It's okay to fall a little behind, you don't always have to be first or the strongest. Also think before you act a little more, but her present self is still learning that herself. Tsume's determined to become the best weapon user of her generation, and possibly many more. Her future is set on that one goal.
Yumi and Yume would tell each other that they look cute probably, maybe get out more. Future twins just hope to get on a team and get stronger, each for their own reasons.
Kioko would tell herself that, despite everything, they were doing it. They weren't the strongest Genin in the village and they certainly weren't making waves among shinobi, but they were doing their part. They had won a spar, finished a mission, and even helped to save the Kazekage! The future doesn't look like it will hold a ton of fame or praise, but it will hold people who she has helped. That's why she became a shinobi to begin with.
Saya would tell herself to be more open with Maia, to be less attentive to Seto, and to keep a closer eye on Rei Uchiha. She doesn't regret any of her past decisions and would easily make them again. All in all her path has been one she's mostly content with. Her future is blurry currently. The change of Hokage, the attacks on the village, the war. It all makes it difficult to determine what comes next.
Haruko would tell herself to stop pretending. People won't judge her just for being a samurai, they won't look at her strangely because of her grace. Kumogakure is a home worth defending, filled with people who would openly accept her. Also to get used to never having alone time. Her future is one likely steeped in warfare but it is her duty to follow the Raikage's orders.
Ty Lee would tell herself so, so many things. It's uncertain if she would say to avoid becoming a Jashinist. It's been a point of contention and conflict in her life, but it's safety, it's home. Even when she's alone she has that. She would tell her to be more understanding of Kanbe and his hesitance toward the religion. She would tell her that heartbreak is coming and when it comes things will get harder than she could imagine. She would also tell her that it was worth the memories and to hold on to them as tightly as she could. She shouldn't dwell on the hurt or the pain, but remember the times that made losing people she cared about hurt in the first place. She has a long, long time to hurt, but only a short time to love those around her. Never waste a moment, never take a single breath they take for granted. They only have their lifetime, do everything you can to make it a great one for them. The future is something she never really thinks about in a positive light. Instead she'd rather think of now.