Tap (or swipe, if you're using full-screen gestures) the Recents button on the nav bar. It looks like three vertical lines.
Swipe in from the right edge of the screen to see your current app. By default in One UI, when you tap the Recents button it pulls up the app opened before your current one.
Long-press the app icon at the top of your current app's preview until a menu appears.
Tap Open in split screen window.
The first app will appear in the top sliver of the screen, with the recent apps menu filling the bottom 3/4 of the screen. Swipe through the recent apps to select one or tap the Finder search to look for an app that hasn't recently been opened.
Type in the name of your desired app.
Tap the app you wish to split-screen on bottom.
The two apps will split the screen 50-50 to start. To resize the apps, drag the blue divider line up or down to your liking.
