Two ways to place: tap a player then tap a spot, or tap a position first (it shows Selected) then tap a player. Tap one position then another to swap them. Drag also works. The red circle adds a player to the batting order even if they're not fielding.
Batting Order 0 batting
Use ▲▼ (or drag on desktop) to reorder. Base coaches aren't added to the order automatically. × drops a player from the order but keeps their spot on the field.