I need your advice.

I got a non immigrant O one entry one year visa in June of this year. It's the first time I'm in Thailand with this kind of visa.

Can you please tell when exactly I have to go to immigration on Koh Chang to extend my visa for further 90 days. On the 60th day after entry or in the 90th day?

What documents do I exactly need to change my visa to get a multiple entry non immigrant O. The bank guarantee, the bank book, the residence confirmation, photocopies of my passport and the stamps of the immigration office and what else?.I know I have to present myself 15 days before the 90 days exceed to apply for changing the visa.

Your advice is highly appreciated.