Rehearse one agreed attacking pattern end to end so that the shape, the timing of the runs and the distances between units become automatic. Every player should know where the ball is going next and be moving before it arrives.
Set the attacking unit out on cones in your match shape on a full pitch, with an opposition keeper in goal and no other defenders. Start every rep with the same pass from the centre-back and run the agreed pattern through to a finish. Players return to their cones and go again. Insist on match speed and match distances: the value of shadow play is lost the moment it becomes a walk. Once the pattern is automatic, name a different one before each rep (outside overlap, inside overlap, central combination, cross and finish) so players have to recognise which is on rather than run one script. Progress it by adding a defensive unit and turning it into a phase of play.
Shadow play installs a shared picture. That is its entire purpose and it is a real one: eleven players cannot coordinate movements they have never seen, and there is no way to establish where everyone will be during a given attacking pattern while eleven opponents are actively preventing it. Removing the opposition removes the only obstacle to learning the shape.
The non-negotiable is match speed and match distances. A pattern walked through at half pace across a compressed area teaches timings that do not exist, and players who have rehearsed at seventy percent will arrive late when it matters. If the runs are not being made at the speed they would be made on Saturday, the rehearsal is actively installing the wrong thing.
Moving before the ball arrives is the specific habit being built. In a rehearsal everybody knows where the ball is going next, which is exactly the condition that lets a player set off early and feel what the timing should be. The overlap should be running as the winger receives, not once he has turned, and that is far easier to establish here than in a game.
The honest limit is that this teaches positions rather than decisions. Players learn where to be and learn nothing about reading an opponent, and a team that lives on shadow play produces beautiful attacks against defences that behave as expected and freezes against ones that do not.
So time-box it. Fifteen or twenty minutes to install, then put a defence in and let the pattern survive contact or be adjusted. Rehearsal without opposition afterwards is choreography.
Save it to your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block. Or start from a blank pitch and build your own.