It is very simple and quick. But you have to have local Indonesian Driving License already. The International Driving License itself was administered by IMI (Ikatan Motor Indonesia/Indonesian Motor-machine Association), but now it is administered by NTMC Mabes PolRI (National Traffic Management Center - Indonesian Police Headquarter).

The NTMC Mabes PolRI office is located at Pancoran, near Menara Syaidah, if you ask Taxi driver to go there, there will be no difficulties at all.

Now what to prepare?? Just these things:

• 1 pcs materai/stamp duty
• 3 pcs pass photo 4×6 with blue background and formal shirt
• KTP (for Indonesian) and its 2 pcs copies
• Driving License (SIM) and its 2 pcs copies
• Passport and its 1 pc copy
• KITAP (for Foreigners) and its 2 pcs copies
• Administrative Fee: New Rp. 250,000; Extend Rp. 225,000

The International Driving License itself valid for one year and can be extended.

That's it, wait for maximum 2 hours and you will have your International Driving License ready. Plus the Polwan (Polisi Wanita/Female Police) usually quite cute and friendly ;) Right, those Polwan as seen on TV on Traffic Report/Farah Quinn's show.

Hope this post helps.

*I have done this long time ago, but just have time to write this today*

UPDATE: KITAP also needed based on recent comment from JM (see comment section)