Put the tire with the new tube inside back on the rim.

Inflate the tube slightly to make it easier to insert inside the tire prior to attatching the tire and tube combo back on the rim.

Make sure the hole in the rim lines up with the valve on the tube properly.

Avoid a misaligned valve. This can cause the valve to break internally as the rubber will be cut around the valve stem by the wheel where the valve protrudes from the rim.