Anyone has experience about this? Would you please tell me about the process? Is this similar to filing a new H-1B? What docements should I provide? Thank you very much!

Thanks a lot! One more question. If the approval date is after the expired date, can I still work in that period?

for extension, once u submitted your application, u get a 240 days grace period automatically, that's for protection of u in case INS(what's the new name?) is really slow. and your extended H1(3 years) will start from expiration date of your first h1 period.

It's real simple. don't worry.