The procedure to apply to Ecole Polytechnique is simple. Candidate can only apply online. There are two sessions to apply either spring or autumn session, but for Cambodia, the juries from Ecole Polytechnique will pass for spring session. The application is open from July to September.
As of spring 2020, Ecole Polytechnique has joint with ENSTA, ENSAE, Télécom Paris, and Télécom ParisSud in building the INSTITUT POLYTECHNIQUE DE PARIS (IP PARIS). All international students could benefit from the same application process and exam in order to get a chance of being admitted to one of the schools listed above.
The preparation of documents are required as indicated in the following link. This link shows all necessary and precise steps to follow : Application to Ecole Polytechnique.