These instructions are if you want to be connected to the internet but still pretty private.
The ideal is to install LineageOS. If that's not possible on your device, then when setting up your device, don't create a Google Account and don't use the Google Apps or Google Play Store or cloud storage.
To sync your files with your computer, including pictures taken, videos taken, music, podcasts, and books, use Syncthing. Alternatively, you could sync them manually by plugging in your phone with a wire, but that's less convenient because you have to do it manually, whereas Syncthing continually syncs your files automatically.
For apps, you can install F-Droid and get a lot of open source apps from there. This prevents the common problem of using apps that track you and are ad-supported.
For web browsing, there are many options available through F-Droid. I like the Fennec web browser (a version of Firefox).
For syncing contacts and calendar, I decided to just use them on the phone, and back them up instead of syncing them to computer-accessible software. I had previously used Decsync to sync them with my computer, and I still use it to back them up by using DecSync to cause them to be automatically converted to regular files, which are then synced with Syncthing. If you have the habit of entering them on the phone, then this is OK. If you already have a way to back up your phone regularly, then you don't need to use DecSync (but the default backup software for LineageOS, SeedVault, currently doesn't backup the calendar, though there are plans for it to do so.).
Another option is to set up a Radicale server on the computer, and connect to it on the phone by installing the DAVx⁵ app; however, that takes more effort to set up and is intended for computers (servers) that are always running. It may do weird things and give error messages if you regularly turn off your computer so that your phone can't connect to the server. With Decsync, it converts the calendars and contacts to files which sync through Syncthing whenever both of them are running.
This process will not fully protect you. To be 100% protected you have to live without a cell phone. But it's a compromise that minimizes, rather than eliminates, Surveillance Capitalism.