This below is the sample code in the demo files of Google Maps APIS v2.
This shows us the HOW TO USE the custom icon in Google Maps.
private void addMarkersToMap() {
// Uses a colored icon. this is default icon.
mBrisbane = mMap.addMarker(new MarkerOptions()
.position(BRISBANE)
.title("Brisbane")
.snippet("Population: 2,074,200")
.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_AZURE)));
// **Uses a custom icon.**
mSydney = mMap.addMarker(new MarkerOptions()
.position(SYDNEY)
.title("Sydney")
.snippet("Population: 4,627,300")
.icon(BitmapDescriptorFactory.fromResource(R.drawable.arrow)));
R.drawable.arrow will be your custom icon : Make your icon using the Graphic software(such as Photoshop)and then insert your icon into the folder of res/drwable in your project.