The app features a cursor widget similar to a tape measure: You first pin the celestial body you are shooting (moon center in this example), and then draw the cursor down to the horizon to read off the angle measured. The widget switches spontaneously to the  horizon mode (above right) when drawn near-vertically below the pin. Use the  auto-align or nudge controls to precisely center the body  inside the cursor perimeter prior to hitting pin. You can use auto-align and nudge  similarly to refine the horizon match.

Hit the sextant icon button bottom left to export your measurement to Celestial Navigation 360. The latter opens directly in a New Sight wizard for the date/time and altitude it has received.

Don't forget to set Dip consistently with the photo as you step through the wizard.  For instance, there isn't an open sea horizon in the above screenshot, the shoreline affords instead a 'horizon short'.  

Calibration check

Internal camera calibration of Sextant Frame relies on manufacturer specs which on some devices may be incomplete or inaccurate. So it's advisable to check it, as is easily done by shooting a moon/planet sight from known position determined via GPS or otherwise: When you set Assumed Position to match your actual position, the sights reduction  intercept becomes purely a measurement error. If the auto-calibration is healthy, this error shouldn't normally exceed 20 NM or so.

Bright stars at nautical twilight

As mentioned above, typical device cameras do not capture stars very well in general. Nevertheless , the very brightest  (Sirius, Vega, Arcturus..) often capture reasonably well at intermediate altitudes against the nautical twilight horizon.

Bubble horizon

On devices with a gravity sensor, Sextant Frame annotates the capture image with  an artificial  ('bubble') horizon, dashed line of the screenshot above.  To snap to the bubble horizon, move the horizon cursor below-outside the image bounds and hit auto-align.
The accuracy of the bubble horizon varies somewhat between devices and conditions.