GoKawiilFirst, there are the core requirements. The update process requires room for temporary files along with the main download, so you'll want to have a good amount of free space. Try to have around 18 to 25GB available, using unconventional ways to clean up your iPhone if you're currently low.
You'll also need enough power to update your device. While 50 percent battery life should be enough to get you through the process, it's much safer to keep your phone plugged in while updating. Keep your phone connected to Wi-Fi too.
Furthermore, you'll need to decide if you want to start the update yourself or let it run automatically. Head to Settings > General > Software Update > Automatic Updates to configure this. If you choose Automatically Install, your phone won't start installing iOS 27 as soon as it's available. Rather, it will update when charging and connected to Wi-Fi overnight. You might get a notification about this, which could be days or weeks after the update becomes available. To start the update manually instead, wait on the Software Update page.
While you're considering this, you should also check if you've updated to iOS 26.6, which includes an indexing update to make the shift to iOS 27's Siri changes smoother.