Begin by tapping the blue '+' button in bottom right corner of the map. Here you'll see all available feature classes. Select the circle type feature class you want to add to your project.
The first step in drawing a circle feature is setting the center point. Pan & zoom on the map to set the crosshairs and then tap 'Set Center'.
After setting the center point you'll have the option to drag a handle to set your circles radius. Radius, Perimeter, and Area values are visible while doing this to help you get the correct size. You can pinch to zoom out on the map if you need more space to move the handle. When you're done setting the size of your circle tap 'Save' to add it to your layers.
If you know the Radius, Perminter, or Area of the feature you're creating you can manually enter these values by tapping the text fields at the bottom of the screen. When you change one value, the others will automatically be updated.
Select the feature you want to edit and tap 'Move' to move the center point of your circle. Pan and Zoom on the map to find your new desired center point and tap 'Move'.
Select the feature you want to edit and tap 'Edit Size' to change the dimensions of your circle. You can drag to set the new size of your circle or manually enter values using.
You can delete a feature by selecting it and tapping 'Delete' at the bottom center of the map. You can also delete features from the Layers dialog.