I was on h1b from oct 2012 to oct2016. I have B1/B2 visa. until 2027. How can I start working on h1b if I have a job offer from company ?

Since you are in Canada, the most common scenario is that if you are still there at the time that you secure a job offer from a US company, the company can petition for a new H-1B since you did not use up the entire six years maximum stay from your prior H-1B employment. Once approved, you would either visa process at an American consulate in Canada if you are not a Canadian citizen, or simply enter the US with the H-1B approval if you are a Canadian citizen. You would of course have to adhere to the Covid-19 protocols. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.