Lets say I want to spot start a IDP.

Beyond just looking at the projections, how do you go about coming up with a starter.

Bad D against good offense?
D with bad offense?

Gotta think the Bears with that offense will be on the field all day. Same with SF.

Is that how you go about looking for a spot start?

for spot starts I go with matchups. For instance if I have a SS that is great against the run and a FS that gets ints, i will take the SS if it is against a predominantly running team or in a game I think will be low scoring. I take the FS if I think the other team will put it in the air a lot

Pick a LB that plays in a 4/3. Otherwise, pick a SS.
I always look for the players who make a lot of tackles. Interceptions & fumble recoveries are bonuses. You can never assume someone is going to make an interception.

Use this rule, and you can't go wrong. Trust me on this.