It's hard to give an exact time as it depends on the manga's plot development. But typically, Yuta Okkotsu shows up when certain events and character arcs lead to his introduction.
One popular 'bnha deku hurt fanfic' is 'Bleeding Hearts'. It delves deep into Deku's emotional and physical pain after a particularly tough battle. Another is 'Scarred Hero' which shows how Deku copes with the long - term effects of his injuries on his mental state.
First, you need to understand Deku's character really well. Know his strengths, weaknesses, and what makes him tick. Then, think of a scenario that would cause him harm, like a powerful villain attack. Outline the story, starting from the incident that hurts him, how he reacts, and how others around him respond.
Well, if Fubuki and Tatsumaki were Deku's sisters in this fanfic, it would completely reshape the My Hero Academia world. Deku's character arc might be influenced by his sisters' powers and personalities. Fubuki, being from One Punch Man universe, could bring in her esper abilities and her leadership skills from the Blizzard Group. Tatsumaki, with her extremely powerful psychic powers, could act as a sort of protector for Deku. Their presence might also change Deku's relationships with other characters. For example, Bakugo might view Deku differently if he had such powerful sisters. It could also lead to new storylines involving the different universes colliding, like how the rules of the My Hero Academia world would interact with the One Punch Man world elements brought in by Fubuki and Tatsumaki.
The dynamic would be really interesting. Deku is known for his kind - hearted and determined nature. Fubuki, being a confident and somewhat prideful character, might initially see Deku as a bit of a weakling but would gradually come to respect his growth. Tatsumaki, who is very powerful and has a bit of a short temper, might be overprotective of Deku at times and get frustrated when he puts himself in danger. Deku, in turn, would look up to his sisters and try to learn from them. He might also try to mediate between Tatsumaki and others when her temper flares up.