You need to have following valid documents
1) Registration of your vahicle
2) MVPI (Motor Vahicles Periodic Inspection) Report
3) Insurance
4) Iqama
5) Driving License
make sure Items no 2-5 are not expired , however if the Registration has been expired they will charge you some fine and will accept your case.
Preparing file for Istamara Renewal
Try to prepare a file and attach orignal copies of items 1,2,3.And photocopies of item no 4,5.
Take this file to any of the showrooms in your city who deal in purchasing and selling of cars and give them your case they will charge their fees and will give you the renewed Istamar in a couple of days.
Note : You may ask your friends about any such showroom to save your time trying to search one :)
