First of all, using a mouse to play PUBG Mobile can get you banned from the game. It is considered a handicap when there are other players using touchscreen and others in the same match using a mouse or a controller. Now to play using a mouse you have to first plug an OTG cable to connect the mouse with the smartphone. The smartphone should automatically detect the mouse and a curser should appear on the screen. All that is left is to open the game and go on shooting.