Takes about ten minutes. You need a trusted friend at the start and at the end.
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Go to Settings > Screen Time > Lock Screen Time Settings and set the passcode to 1234.
It then asks for a Screen Time Passcode Recovery account. This is the only time it asks, so your friend needs to be there now. They enter their own Apple Account, not yours. If your account is on there, you can tap Forgot Passcode? later and reset everything. Tapping Cancel doesn't help, it falls back to your account anyway.
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Go to App Limits > Add Limit. The report on the main Screen Time page shows which apps you use most. Pick those.
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Turn on Block at End of Limit for every limit. Without it, you just get an Ignore Limit button you can tap.
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Optional: if you don't want these limits on your Mac, open Screen Time on the Mac and turn Share Across Devices off. Do it now, you can't after the next step.
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Go to Change Screen Time Passcode, enter 1234, then hand your friend the phone. They type four random digits and don't tell you.
Done. You don't know the passcode, and Forgot Passcode? asks for their Apple password.
Why 1234 first?
Two reasons. Block at End of Limit only shows up once a passcode exists, so you need one before step 3. And if your friend set the real passcode at the start, you'd have to call them over every time you added a limit.
The catch
Your friend controls your settings now. If they cave, or you lose touch, you're stuck. Pick someone who will say no, and tell them in advance you'll ask.
No friend to ask?
WaitToUnlock does the same thing without one. It generates a passcode, walks you through entering it so you can't memorise it, then hides it for 6 hours.