You mark a spot on the ground, and make sure that as you're rotating the phone, the weight is always over that spot.

Another technique you could consider is using a plumbline attached to the phone (a trick known as a philopod). The easiest way to do this without error would be with a tripod and a special panohead (see: How are virtual tour photos taken?) with a tripod adapter to hold the phone (or maybe a special smartphone setup), but this might be more gear and expense than you want to go to. The trick is not to move the camera in space, but to rotate around the lens's no parallax point. It can be relatively easy to correct in others (shooting outside with no subject matter particularly close by).

This can be very very difficult to correct if you're handholding the phone (camera) in some situations (i.e., shooting inside a small room). It could be that you missed covering that part of the scene (blank screen no data), and it could be that you created some parallax error by moving the camera in space, rather than rotating the camera's lens around its 'no parallax point' (sometimes erroneously called the nodal point). What you're running into, in terms of the places where the photos don't join up seamlessly, could be one of two issues.

Cell phone photosphere tripod