All of the resources that I can find say that it should be done with the screw tip pointing up and the hanger below with up to a twenty-degree angle from horizontal.
Contrary to what you might think, the best angle for the screws is slightly upward, meaning the hanger is slightly lower than the teeth in the ice. This counterintuitive method is better because the holding power comes from the threads themselves and not from a “snow picket” effect, meaning you don’t get any mechanical advantage having the shaft of the screw levered against the ice (Fig. 2).
How to Build Bomber Ice Anchors
More surprising were Harmston's findings about screw angle. As it turns out, screws hold much better when they're angled in the direction of the falling force, as in the second scenario above. Perhaps needless to say, this is not intuitive. Much of the force is then resisted by the threads rather than the girth of the screw itself. This isn't a small difference either: an ice screw placed at a downward angle is as much as twice as strong as a screw placed at an upward "negative" angle.
Gear Physics: How Ice Screws Keep Ice Climbers from Falling to Their Deaths
Limited testing by Chris Harmston of Black Diamond and Craig Luebben about 10 years ago showed that ice screws with the hanger end angled down (negative angle) up to 20 degrees were stronger than screws with the hanger parallel to the ice or angled slightly up (positive angle). A negative angle does not, however, increase the strength of the threads; rather, it decreases the chance that the ice will blow out from under the screw.
(This article links to the actual 1999 research report by Chris Harmston)
Should I Angle Ice Screws Down?
It was once thought necessary or beneficial for ice-screws to be place horizontally or with the hanger up for optimum hold. However, it has since been found experimentally that a screw place with the tip angled up often holds as well or better. This surprising result is thought to be due to the previously underestimated role of the threads in holding the screw in place. However, horizontal placements are usually recommended.
So you would want to angle the tip up into the ice with the hanger pointing down.