How should I set my fantasy football lineup each week?
Start players based on their role, projected workload, matchup, status, weather, and league scoring format. Start anyone who has been designated as active and should receive consistent playing time. Don't sit a good player just because the opponent is tough. Review injury reports and inactive lists before kickoff. Use projections as a tool but don't let them make your decisions for you.
Availability: Make sure everyone on your roster is expected to play. Pay attention to those who are listed as questionable or limited in practice. Inactives are usually released late, but you can find out who is likely to be active by checking trusted sources before each player's game. This may help you avoid a 0 score that you could have prevented.
Then check out their role and how many snaps they're likely to see. A RB with a defined number of carries and targets is generally a better bet than a RB who has to rely on one huge play. The same goes for a WR who plays a significant number of offensive snaps and runs plenty of routes, or a TE who's regularly targeted. Fantasy points in recent weeks are also something to take into account, though the chances behind those points may mean more.
Matchups can help you choose between players in similar categories but aren't the only consideration. You shouldn't start a mediocre player over a good one because he has a better matchup. If a solid player is facing a tough defense, he's probably still a better option than a low-volume spare part with an easier matchup. But look at how the defense plays, the expected tempo of the game, the potential game script, and whether the team will likely run or pass more than normal.
Your scoring format also dictates how you compare players. QB rushing, reception points, bonuses, turnovers, TE bonuses, etc. In a full PPR league, targets are extremely important. In a standard league, yards and TDs might be more important.
Ask yourself if you want a safe score or a player that has a higher upside. If you're the favorite to win, maybe a consistent player with a locked-in role is the best pick. If you need a huge score to keep up with your opponent, maybe a more high-variance player with big-play or touchdown upside is the right option.
Once you've done your comparison, don't feel compelled to switch up your lineup if new rumors arise or if you get emotional. Factor in role, health, matchup and projected opportunity to decide. And keep looking at late news, because there may still be changes for games that haven't started yet.