For those who have lost their citizenship and want to become the Indonesian citizen, they will need to apply via 'naturalisation'. It is stated in Article 8 & 9 of citizenship law law UU_No_12_2006 - Indonesian Citizenship Law.
Article 8
Citizenship of the Rep. of Indonesia may be acquired through naturalisation
Article 9
Requests for naturalisation may be forwarded by the applicant upon meeting the following requirements:
a. Aged 18 (eighteen) or married;
b. At the time of forwarding the application, the applicant has resided in Indonesian territory for at least 5 (five) consecutive years or at least 10 (ten) years intermittently;
c. Sound in health and mind;
d. Able to speak Bahasa Indonesia and acknowledges the state basic principles of Pancasila and the 1945 Constitution;
e. Was never legally prosecuted due to acts of crime and sentenced jail for 1 (one) year or more;
f. Upon acquiring Indonesian Citizenship, will relinquish any other citizenship;
g. Employed and/or has a steady income; and
h. Pay a naturalisation fee to the Government Treasury.
I attach the key Information, how to do it (see attached)
Because you will need to meet residential requirement, you can not do it while still living abroad. To get a stay permit to meet residential requirement, there are various routes available, such as apply for E32C first. This stay perm will only last for 2 years but it could be renewed/extended. But the easiest and less complicated option (but more expensive) is to apply for GCI, e.g E32E, with this visa you could stay in Indonesia for 5yr+ (unlimited). After meeting the residential requirement you could then apply fro naturalisation.