Massive multiple-input multiple-output (MIMO) emerges as one of the most promising technologies for 5G mobile communication systems. Compared to the conventional MIMO channel models, channel researches and measurements show that significant non-stationary properties rise in massive MIMO channels. Therefore, an accurate channel model is indispensable for the sake of massive MIMO system design and performance evaluation. This article presents an overview of methods of modeling non-stationary properties on both the array and time axes, which are mainly divided into two major categories: birth-death (BD) process and cluster visibility region (VR) method. The main concepts and theories are described, together with useful implementation guidelines. In conclusion, a comparison between these two methods is made.