Half-PPR leagues are a nice medium between standard and full-PPR leagues. They don't over-value the wide receivers and pass-catching backs but also evens the playing field when compared to bell-cow running backs. A half-point is given to a pass-catcher in addition to their receiving yards/score. Whether that is a 1-yard dump-off on a screen pass or a 45-yard touchdown throw- each player receives a bonus, which makes high-targeted receivers/running backs very valuable.
Elite running backs still matter in half-PPR scoring, however, it allows wide receivers to be taken later on in the draft which become relevant and provides a weekly floor. In standard scoring, there is a rush to be running back heavy early on, compared with an increasing amount of wide receivers are taken in full-PPR leagues. In half-PPR leagues, each owner should let the draft come to them and pick based on the value that presents itself.
If you end up with 3 running backs, 1 tight end, and 1 wide receiver after five rounds, then so be it. As long as you're drafting value in the early-to-middle rounds, you won't be behind the competition during the rest of the draft and can fill in the rest of your starting lineup later on.
Figuring Out the Flex
Generally speaking, the flex roster spot should be filled by your best player based on the matchup in a given week. In any sort of point per reception league, the tie more often than not goes to the receiver, as they're more likely to score more points due to the extra bonus given for making a catch. The same situation applies in half-PPR leagues, although strong consideration can be given to running backs whose projected target totals have increased given the right matchup.
Now this isn't to say you should be drafting the 3rd wide receiver on the Jets over, say, a pass-catching back like Tarik Cohen later in the draft just because wide receivers generally will put up more points than running backs. Pass-catching backs provide a lot of value and do set a weekly floor for your team.
Player Movement in Half-PPR Leagues
Running backs who aren't relied on and don't contribute to the passing game much at all during the season find themselves sliding down draft boards each year. This season with the wide receiver position, we find boom-or-bust type guys who haul in long touchdowns and only have a couple of receptions per game having their ADP increased in half-PPR leagues.
Players to Downgrade Slightly in Half-PPR
Don't be afraid to draft a backup running back who contributes regularly for their team. When it's third down or when a team is trailing big (or late) in a game, these are instances when they can emerge. Many late round gems can be found in the middle-to-later round of drafts to supplement your initial roster, given the benefit of a half-point per reception.
Players to Upgrade Slightly in Half-PPR
What do you do differently on draft day when selecting a QB in half-PPR versus standard? Nothing. In half-PPR leagues, you should still follow the Late-Round Quarterback strategy and fill up your roster with quality running backs and pass-catchers who are projected to see a heavy dose of targets, before jumping in on a QB. This will ensure you will have tons of depth to play matchups or incase players get hit with the injury bug.