where can i find steps to back up iPhone to time machine?

You can't because there is no way to do a backup from the iPhone to Time Machine directly.

The closest thing you can do to come to that would be to backup to iTunes and then use Time Machine to backup your iTunes library.

That's a two-step process.

1. Click here and use the instructions to back up the iPhone to a computer.

2. Back the computer up to Time Machine.

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It's not directly possible.

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can't back up iPhone to time capsule? only MACs?

You can also back up to iCloud.

See this support document for more details on backups:

Back up and restore your iPhone, iPad, or iPod touch using iCloud or iTunes - Apple Support