You'll want to read the entire article but here's a quick summary:
- Pick a D/ST that faces a team that passes a lot. This increases the odds of getting interceptions -- including pick sixes -- and sacks. This is in the same vein as RBs getting carries/touches or WRs getting targets: more opportunity to score.
- Use the Vegas line to help pick your D/ST.
- Try to use a D/ST that is on a team favored to win.
- Try to get a D/ST facing a QB with a low completion rate and/or high sack rate.