Simply connect one number to the highlighted number by smoothly drawing a line on the screen. There are two connection types, viz., serial and random. You should not touch the wrong number, the boundary, or the lines already drawn, and you should also complete the level before the time's up.
It is very easy to play this game with a phone stylus (touch pen for Android phones). You can make a stylus with a few materials at home, such as a small stick, cotton, a small piece of aluminum foil or copper wire, and water.