There are plenty of creative possibilities for this assignment of a journal entry about being stranded on an island. Since the prompt focuses on the word “alone,” you could focus your entry on what it feels like to be alone on the island. You might write about how you talk or sing to yourself, how you have become closer to your faith or spiritual tradition, or how journaling has helped you to fight the loneliness.
You can also write about how you have learned to fend for yourself on the island. Think about how you would get your food, perhaps by fishing and/or hunting, by scavenging, or by growing a garden (or all of the above). After twenty-five days, you will probably have built a shelter for yourself, so you could describe that.
Further, you might talk about the dangers you encounter on the island. If there are wild beasts, you can describe what you are doing to defend yourself from them, or perhaps to befriend them. If there is nasty weather, think about how you would protect yourself.
Finally, after twenty-five days on the island, you would probably be thinking about your chances of rescue at this point. You could talk about your emotional state, whether you are hopeful that someone might find you or whether you have given up on that and are resigning yourself to living for the rest of your life on your island.