iPhone

1. Scroll down to the + icon and tap it
2. Swipe through the available panels
3. Tap the one you want or tap elsewhere to cancel adding a panel

iPad

On iPad you cannot add additional panels, only replace a panel. This can be done by:

1. Tapping on the panel to bring up the panel's options
2. Tapping the plus (+) icon
3. Scrolling through the available panels and selecting the one to replace the old panel with