Begin with brainstorming some really cool fantasy elements. Magic spells, enchanted forests, or floating islands are good starts. Decide on the format of the choices. Will they be simple yes - no, or more complex multiple - choice? After that, write a rough draft of the beginning of the story. It should introduce the setting and the main character in an exciting way. As you write, keep in mind that the reader's choices should feel meaningful and lead to interesting and different parts of the story.
Well, you need to have a great concept. Think about things like a hidden realm where only those with a special key can enter. Start by outlining the basic plot. You know, like what the overall goal is for the protagonist. Maybe it's to find a lost magical artifact. Then, start building the scenarios for each choice. For instance, if the character chooses to go left in the forest, they might encounter a friendly elf who gives them a clue, but if they go right, they face a tricky trap.