By a position player, I’m assuming that you’re referring to an infielder and outfielder. I’m also assuming that you’re not referring to a backstop position.
First base is particular to judging incoming throws from infielders mostly. As such, the first baseman uses a mitt, not a glove. This mitt is awkward at best for fielding grounders coming off the bat, yet, takes considerable practice “scooping” throws out of the skins. Range of motion is somewhat “if-ee” at best, so I would concentrate on giving fielders a good target with the mitt, and tons of practice “scooping” balls coming at you with a lot of zip. In addition, being a cutoff man (sometimes) from right field on the throw takes a ton of game scenarios and understanding time and distance with respect to baserunners and where the ball is.
Second, and short require cat like reflexes and spot on judgement, soft hands and willing to give one’s body up to block grounders from going into the outfield. So, Concentration on leg and lateral movement is the thing here. Coupled with that, practice throwing slightly off balance, feel the ball right after possession for the seams that says… “four seam grip”, thus reducing the curveball effect when throwing to first and home. Range is super important here at these two positions and gauging the “hop” of the ball. Take a tennis ball to a wall and bounce the ball off the wall starting at 30 feet. Move closer and closer to the wall, throwing at the same rate of speed, until you can control the fielding from 15 feet away. This is tricky, so don’t underestimate this drill. Also, try this drill without a glove so as to give you the “feel” for possession - soft hands and control the grip. The shortstop is suppose to be the athlete of the fielders. Superior physical shape and a keen eye for plays prior to and while developing. Controlling the left outfielder so he/she doesn’t rifle the ball into home plate on impulse, is a common expectation for the shortstop.
Third requires amazing eyesight and quick lateral movement to the right and left of the bag in short bursts. A howitzer of an arm to reach first is a given. Weak arm? Stay away from holding down third. By the way, there’s a reason why third is called the HOT SPOT. When the ball comes off the bat and at third … it’s screaming. Third baseman can actually hear the seams hissing through the wind sometimes.
Everyone of the outfields must have range with their legs, keen eyesight for judging balls in their direction and exceptional line of sight judgement for night games. Night games are particularly difficult because the lights tend to eliminate a sky background in addition to removing shadows for the purpose of perception.
So, every fielding position has different talents that require a particular need, physically and mentally.
As a general conditioning statement, I would suggest hitting the treadmills, go a few rounds with a speed bag, jump rope, toss a tennis ball again a wall - A LOT, play the game and study the game at the same time.