you don't but based on the time frame you have given you will not have enough time to get the extension and will need to start the whole process from scratch. Not to mention if you are caught teaching without a work permit you will be deported and banned from thailand for a minimum of 5 years and could face prosecution too.
For the first year extension you will need the 400k in the bank deposited 2 months prior to applying for it because the British embassy no longer certify income. For the second year, if you deposit ฿40k+ every single month you can obtain the extension and further extensions that way instead.
Can't get a work permit without the transcripts being notarized and then authenticated by a Thai embassy in your country. Without a work permit you cannot legally work in Thailand.
Also there is no 12 month non-b, you get a 3 month non-b and then if you meet the requirements you can get an extension of stay which is 12 months. Your certificates would not be ready for the extension as all paperwork needs to be done and given to immigration prior to the 3 month mark.