First, come up with a couple. Decide on their personalities, like if one is more shy and the other more outgoing. Then, think of a situation where tickling might start, for example, during a playful pillow fight. Just start writing and let the story flow as you imagine how they would react to each other's tickles.
Well, start by creating your characters. Give them names, backstories, and a relationship dynamic. For the tickle part, you could describe the sensations, like the light touch of fingers and the laughter it elicits. You also need to build a plot around it. Maybe they are having a tickle fight as a way to relieve stress from their daily lives. Use vivid language to make it interesting, like 'her fingers danced across his ribs, sending shivers of laughter through him'.
To write a 'couple tickle fanfic', you should begin with a strong concept for the couple. Are they newly in love or have been together for a long time? This will affect how they interact. When it comes to the tickling aspect, consider the emotions involved. Is it a form of flirting? Or a way to comfort? Then, structure your story. Have a beginning that sets the scene, a middle with the tickling action and the couple's reactions, and an end that ties up the story, perhaps with the couple sharing a tender moment after the tickle - filled encounter. You can also add some dialogue to make it more engaging, like 'Stop it! Oh, but it's so much fun!' as they playfully tickle each other.