First, develop your OC well. Give them a unique personality, backstory, and appearance. For example, if your OC is shy, explain why. Second, understand Sans' character from the original Undertale. His laid - back yet observant nature should be reflected in the interactions. Third, create an interesting plot. Maybe your OC has a special power that affects the Undertale world in a new way. Also, use good grammar and spelling to make it a pleasant read.
To write a good Sans x OC fanfiction, start with a strong concept. Think about how your OC will fit into the Undertale universe. For the relationship with Sans, build it gradually. Don't rush into a romantic relationship right away. Show their friendship first. Add some conflict, like an enemy that threatens both of them. And don't forget to add some Undertale - specific elements, like the different areas in the Underground and the other characters. This will make the fanfiction feel more authentic.
Chapter 416, "Such a Sensitive Nose, Born in the Year of the Dog?", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Yu Li's words successfully choked Lady Nina.
Chapter 467, "Your Boars Have All Run Back!", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. A useful checkpoint is the way the scene starts with He had recently been meditating on how to accomplish the tasks...
Chapter 428, "Stunning Debut!", is the clearest early answer because it turns Stunning Debut's situation into a concrete plot event. The chapter turns that into something concrete by moving from I'm going to perform a cover of the song 'The Ocean'..." Jiang Shixu...
Chapter 480, "I'm the Guest, Give Me 500 Points", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. The chapter gives readers a real answer by opening on Team H delivered the seafood feast as promised.
Chapter 478, "Someone Comes to Bully Me?", is the clearest early answer because it turns Cheng Biao's situation into a concrete plot event.
Chapter 653, "Everyone's Champion", is the clearest early answer because it turns Yuanyuan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with As dawn broke, "Wilderness Survival 30 Days" released a notice on...
Chapter 513, "A Public Stage", is the clearest early answer because it turns Xiao Shixu's situation into a concrete plot event. The chapter turns that into something concrete by moving from Everyone was sharing their experiences in a few words, the banter...
Chapter 372, "Finish the Wine and I’ll Let You Leave", is the clearest early answer because it turns Jin Siyue's situation into a concrete plot event. It opens with Jin Si Yue suddenly realized that his wrist was back in proper...
Chapter 608, "Congratulations to Team Q for a Successful Escape!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Ji Yanbai narrowed his eyes, still marveling at the audacity of this...
Chapter 819, "Invitation", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. The chapter turns that into something concrete by moving from What Yu Li said is something these prominent families often do. to Lin Manru's studio also resumed work..