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Honestly, long toss and work on footwork. Really should be able to do by just watching a few video's, breaking down each step barney-style if need be, and then put it all together.
Footwork/mechanics can be done easily in your own yard.
One overlooked OF tool is fielding ground balls. Everyone practices fly balls and such but giving the kids different game scenarios and hitting simple 'base hits' to him and working on charging, fielding and knowing when to charge, hold ground and where to throw/look for the cutoff.
Between each pitcher/batter he should mentally go over what he does if the ball is hit right at him, to his left/right, slowly hit, hard hit, etc.
Was it last post season, or two years ago, a MLB OF in the postseason screwed the pooch. He was playing deep and there was a base hit to him. Forget the exact scenario, but where he was playing and how the ball was hit he had zero chance to throw the guy out at the plate but he charged it like the winning run was at 2nd in a game 7 bottom of the 9th scenario. His wreckless abandon to the ball lead to him misplaying it and ultimately a terrible play losing the game.
I think OF'ers, from most coaches I have seen, dont get the same situational work in that INF does. Many teams work PFP for the INF often, but the OFers get stuck as baserunners and lose out on that important block of instruction.
Fortunally my ole ball coaches we did extensive situational OF work, at times at naseum, but it pays off if only just for that one critical play of the year.