Open the app you want to use
Enter Split View by holding the green window button (top-left of the window)
Select one of the tile options to snap the window to the left or right
Choose the other app from the remaining side (or pick from the thumbnail list)
If prompted, select a second window/app to complete the split
To resize, drag the divider line between the two windows
To exit Split View, hover over the top of either window and click the green button, or drag one window away from the split
To use two windows from the same app, open the second window first, then repeat the snapping steps