The application for JPA PIDN is only open once a year and usually in June. Last year it was end of June. U might want to monitor the updates at Esila (JPA site for trainings & scholarships/loans) and JPA Main Page because u wouldn't want to miss the chance.
How to apply? Just go to the links I provided and u can apply online when the application is open.
Three main requirements for 2nd year students and above:
1) Malaysian
2) Not more than 25 y/o
3) Pass (Medicine, Dentistry, Pharmacy & Vet) ,
Other courses minimum standing of 3.5 CGPA.
Keep updated with Esila and Wish u luck.

Erm I'm not very sure bout what u meant by "walk-in" application. Anyways, I was referring to the old PIDN scheme (not for SPM leavers) for those who are already or who are enrolling for degree programmes in public unis and some specific private unis (MMU,UNITEN,) by using results from matric/STPM/foundation/degree's 1st sem. I almost never heard of "walk-in" application by the way. Only for some rare cases. Maybe someone else could help shed some light on that.