Character development is key. Tails' usually shy and intelligent nature should be shown evolving as he gets closer to Fiona. For Fiona, her unique traits should also be highlighted. The setting can play a big role, like a peaceful meadow or an exciting cityscape. And of course, the build - up to the kiss, with their feelings growing gradually through shared experiences and conversations.
In a fanfiction, perhaps Tails and Fiona are on an adventure to save their world from a great evil. Along the way, they face many challenges and have to rely on each other. During a moment of rest in a hidden cave, Fiona looks at Tails with newfound admiration. Tails, feeling the same, leans in and they kiss, sealing their bond not just as comrades but as something more.
One great Loid x Yor fanfic is 'A Spy's Heart'. It delves into the deeper emotions of Loid as he starts to truly care for Yor beyond their fake marriage setup. It shows his internal struggle between his mission and his growing feelings.