Time the run to beat the offside line. Stay level with the last defender, break as the ball is played, get in behind, and finish past the keeper.
A defender holds the offside line. The striker checks to stay level and onside, then times the run to break in behind the instant the passer plays the through ball, and finishes. Emphasise staying level until the pass, then the burst. Vary the timing and the weight of the pass. Use an assistant or a line to judge onside. Rotate the striker and passer.
Offside is a timing skill wearing a rules problem's clothes. Strikers who are repeatedly flagged are rarely confused about the law; they are moving before the ball, because standing still while a teammate looks up feels like doing nothing. The instruction that fixes it is not be careful, it is break on the pass.
Staying level with the last defender is the starting position and it requires watching him rather than the ball. A striker who watches only the passer drifts, because the defensive line steps up and he does not notice. A glance at the shoulder every few seconds is the same checking habit defenders need, applied in reverse.
Bending the run is what turns a legal run into an unstoppable one. Running straight forward from an onside position means arriving in the defender's eyeline with him facing the right way. Curving from wide or from slightly behind the line means arriving from his blind side, and by the time he has turned his head the striker is past him.
The timing that matters is the passer's foot, not the ball. Breaking when the ball is already travelling is a fraction late, and that fraction is usually the difference between arriving at the ball in stride and stretching for it.
Two honest points. First, this drill needs a real defensive line to be worth anything, because a cone cannot step up. Second, a striker doing this properly will still be flagged sometimes, and a coach who reacts badly to every offside will produce a forward who stops running in behind at all.
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.