Wiping an iPhone before it leaves your hands is a five-minute job that people either do badly or skip entirely, usually because they assume deleting things is the same as erasing them. It is not, and the order of the steps matters more than the steps themselves.
Here is the sequence, and what goes wrong when it is done in the wrong order.
Do not start by erasing
The most common mistake, and the one that causes the most trouble afterwards.
If you erase the phone while it is still signed into your Apple Account, Activation Lock stays on. The next person switches it on and is asked for your Apple Account password, which they do not have. The phone is effectively unusable until you sign in remotely to release it.
Signing out first avoids that entirely. Erasing first turns a simple handover into a support conversation.
The order that works
Back up first. Settings, your name, iCloud, iCloud Backup, Back Up Now. Or connect to a computer and back up there. Do this even if you are certain you have everything, because you will find out otherwise afterwards.
Unpair an Apple Watch, if you have one, from the Watch app. It backs itself up as part of unpairing.
Sign out of your Apple Account. Settings, your name, scroll down, Sign Out. Enter your password, which turns off Activation Lock and Find My. Choose whether to keep a copy of your data on the phone, which does not matter since you are about to erase it.
Then erase. Settings, General, Transfer or Reset iPhone, Erase All Content and Settings.
Remove the eSIM, if you are keeping the number. The erase flow asks whether to keep or delete the eSIM, and the right answer depends on whether the number is moving to a new phone or staying with this one.
Remove it from your device list. On another device, or at iCloud.com, open your account's device list and remove the old iPhone if it is still there.
Erase All Content and Settings is a real erase
Worth knowing why the quick version is genuinely sufficient here. The phone's storage has been encrypted since it was set up, and the erase does not overwrite every block one by one — it destroys the key. Without the key the remaining data is not recoverable in any practical sense, which is why the process takes a minute rather than an hour. So there is no need for third-party wiping tools, no need to fill the phone with junk files first, and no need to run the erase more than once. Deleting individual photos and messages by hand, on the other hand, does not erase anything, which is why the full reset is the step that matters.
The accounts that are not on the phone
Erasing the device handles the device. It does not handle the services that were signed in on it, and a few of those are worth a minute each.
Two-factor codes. If the phone was generating authentication codes for other accounts, either through an authenticator app or through iOS's built-in verification codes, those move with your Apple Account restore only if they were synced. Check that you can still get into anything important from another device before you erase.
Messages and iMessage. If you are moving away from iPhone entirely rather than to another one, deregister iMessage, or texts from iPhone users will keep going to a phone that no longer exists. There is a deregistration page on Apple's support site for exactly this, and it is the single most common problem people hit after switching platforms.
Anything with a device limit. Some subscriptions and streaming services count devices, and an old phone can quietly occupy a slot. Removing it from your account is easier while you still have it.
Saved cards in Wallet. Signing out removes them from the phone. It does not cancel anything, and there is nothing to do beyond confirming Wallet is empty before it leaves.
What to do before you hand it over
Check three things after the erase completes.
The phone should boot to the Hello setup screen. If it asks for an Apple Account password at any point, Activation Lock is still on and you signed out incorrectly.
The device should no longer appear in your Find My device list.
And if you are selling it, do not include a case or a screen protector you have been using unless you have checked there is nothing tucked behind them. This sounds trivial and it is the single most common thing people forget.
If you have already erased without signing out
Recoverable, and easier than it looks.
Sign in at iCloud.com or use another device signed into your Apple Account, open the device list, find the erased iPhone and remove it. That releases Activation Lock remotely.
Do this while the phone is still yours if you possibly can. Doing it after the phone has been handed over means the buyer sits at a locked setup screen waiting for you.
Quick answers
Does erasing remove my eSIM?
It asks. If the number is moving to a new phone you usually want to keep it during setup transfer and remove it here; if you are unsure, check with your carrier before erasing rather than after.
What if the phone is broken and will not boot?
Remove it from the device list at iCloud.com. That releases Activation Lock without needing the phone to work.
Do I need to erase before a trade-in?
Yes, and reputable trade-in programmes will tell you the same. Do not rely on them doing it for you.
Is a factory reset the same thing?
Erase All Content and Settings is the reset that matters. Reset All Settings, which sits near it in the same menu, does not delete data and is not a substitute.
Where to get it
For the phone you are keeping, the lock screen is worth setting up properly from the start. DON'T Touch Phone Wallpapers includes contact-line designs where you can put a way to reach you on the screen, which is the part that helps if the new phone ever goes missing.
Sign out first, erase second. Everything else is detail.