No you don't need to fill any forms for any IIMs. If you remember while filling the CAT form you must have been asked to select various IIMs programs you are interested in taking admission. All the IIMs and programs you ticked at that time only makes you apply for IIMs.

After CAT exam your marks, percentile and all the personal details will be send to all the IIMs in which you have applied while filling the CAT form. Then every IIM makes a merit list to called students for next round of interview and WSAT which is based on your aggregate score which comprises of many factors such as CAT marks, 10th 12th and graduation percentage, work experience, engineer or non engineer, gender with different weightages for different colleges.

After the second round again a new merit list is made including interview and WSAT marks which generally have highest weightage in final list along with CAT marks.

Based on this list finally students are given admission.

But yes remember for all other colleges which give admission on the basis of CAT marks, you need to apply individually.

All the best for your preparations and exams!!