In Astro-photography, we typically want to focus the camera to infinity. This is not as easy as it sounds since most smartphone lenses don't have a calibrated focussing mechanism. Moreover, you might have an additional lens or mirror in the light path, which will affect the focussing. Hence, simply moving the focussing slider towards infinity will not ensure good focus.
A Bahtinov mask would have made focussing easier, but making one for a smart phone camera is difficult, since the aperture is usually too small to make a mask. Instead, we have to manually eye-ball the focus and here's a guide to getting good results.
Keep the phone on a steady mount
A steady mount is crucial not just for focussing but for all Astro-photography.
A tripod, or a telescope mount, or even propping against a rock will do.
Choose a bright star
To minimise the effects of camera shake, seeing and trailing, choose a bright star and reduce the exposure time as much as possible. If needed, bump up the ISO instead of the exposure time.
Double tap to zoom in
In SkEye Cam, when you double-tap on the image preview, the view will zoom 4x on that part of the image. You can double-tap again to reset the zoom. You can also use two-finger gestures to zoom and pan the image.
Now, zoom-in on the bright star, and adjust the focus slider till you can verify the star appears as small as possible.