Decide its main job
An island may provide preparation space, deep drawers, casual seating or a place to serve food. Giving it one or two clear jobs keeps the design focused.
Services add complexity
A sink or hob can be fitted in an island when the room and services allow. This needs more planning than a simple preparation island and may affect the floor and extraction.
Measure the clearances
Allow for open appliance doors, seated people and routes behind them. A peninsula or breakfast ledge may work better in a tighter room.