How do stay in Singapore for 6-12 months?

This will not work as you planned.

1. To do work of any kind in Singapore, you need a visa allowing work. If you are self-employed, you need an EntrePass - requirements are too high for a temporary set-up like yours, but there is no other legal way to do this.

2. Renting residential property requires a residence (meaning long-term) visa. Without one (e.g. on a Social Visit Pass or tourist visa) you are limited to hotels, hostels and serviced apartments, which will cost you a minimum of S$3000/month per person.

3. Given that the average visitor stays 3.5 days, you will be seriously questioned about your reasons for such a long stay on your second or third re-entry. If you don't present your case well, immigration will (at their discretion) only give you a few days visa to be able to pack your things and leave for good. Under no circumstances will you be able to stay more than 89 days in any 6 months period on a visitor visa.

The rules are pretty similar in most Southeast Asian countries (there isn't a way to officially do temporary freelance or self-employed work in any of them), but the chances of doing it under the radar are higher in some places. In Singapore, DON'T!

OK. So what is the best way to stay as long as possible - around 9 months. Is there a possible way to just be on a holiday visa for an extended period? or some other way.

Tourist visa are available for stays up to 89 days and allow no work whatsoever.
If you are student and from an eligible country, you could try the Work Holiday Visa (WHV) - the rules are not clear about if freelance work is allowed or not, so you'd have to ask the authorities (running a business is definitely not o.k.).
I am not aware of any other way.