How to buy an iPhone the moment it is back in stock

A restock at a store often lasts minutes. Prepare in advance and it takes under a minute from the notification to a placed order.

A day or two before

Sign in to your Apple ID on apple.com in the browser you will buy from, and every 2–3 hours check you are still signed in: open your bag or account.
Fill in your billing details and save a card to your account, or set up Apple Pay. Typing a card number during a restock costs you a minute.
Add the exact configuration you want (model, colour, storage) to Favorites on apple.com: when it restocks, checkout starts from there in a couple of taps.
Set up an alert for the model on InStock.watch and make notifications loud.

Tap “Buy” right in the notification: apple.com opens on the exact version, colour and storage already selected.
Add to bag and go straight to pickup: choose the store from the notification.
Pick a less popular pickup time: weekday mornings soon after opening are usually calmer than evenings and weekends.
Do not leave the tab open “for later”: stock is checked at checkout, and someone else may take the phone while you wait.

Check the store log to see when the model usually appears, and be ready at that time.
Add nearby stores or the whole city to your alert for better odds.