Examine a map and determine what ocean you will place your island in. This choice will affect many of your subsequent choices, as it can influence things from population density, trading, weather patterns and temperature.
Research conditions in that ocean. Take notes about water temperature, depth, currents and weather patterns. Find out how many islands are currently in that ocean and what conditions are like on that ocean. Map out where the underwater mountain ranges, coral ridges and fault lines are.
Draw your island on a piece of graph paper. Determine the size you want your island to be and then calculate how many square feet each square on your graph paper will represent. Write this as a scale at the bottom of your page.
Draw the individual features of your island. Use the information you researched about the climate to determine what the geography of your island will be. For example, a forest of pine trees would work well on an island in the northern Atlantic, but not so well in northern Oceania.
Make a 3D model or a diorama of your island. Make the model to scale using painted polystyrene foam as a base or a piece of particle board. Build up the model using clay, craft foam, stones or found objects.
Create charts that show the island's temperature, average annual rain fall, population density and natural resources. If the island is inhabited, determine what the people are like who live there. Describe physical characteristics, languages, social structure and economic structure.
Develop a timeline and history for the island including its name and any name changes it might have had in its history. If the island is inhabited, create a flag for the island and determine what the culture would be like. Consider creating a mythos inspired by the island's geography and location or composing a song with music that could be made from materials on the island.