Here’s a short video of my friend Roger Strong demonstrating how to place a good ice screw in waterfall ice.

Keys to good ice screw placement:
1.  Find solid ice.  Chop or clear out any rotten or sunbaked ice (usually white or gray in color).
2.  Find (or chop out) a flat or concave spot.  Make sure it is about 8″ in diameter so the screw handle turns freely around its full radius.
3.  Use a negative angle.  Drill the screw upwards at about 10 degrees into the ice.
4.  Ensure that the screw is fully in the ice.  If not (e.g. bottomed out on rock), use a shorter screw for the strongest placement.
5.  Clip into the hanger.  Tying off screws dramatically reduces their strength.  Chop away any ice that may cause the carabiner gate to open, reducing its strength.

Nuances for glacier ice:
1.  glacial ice is softer/weaker than waterfall ice due to the way it forms.
2.  Using a positive angle, positioning the hanger of the screw 10 degrees away from the potential forces (up if you’re leading).
3.  Screws melt out really fast in the sun, consider using a v-thread for anchors since they melt out a lot slower.