Hello Liangtwn,
Here is my answer assuming all your money in 401k account is pretax.
1. IRS do not have any set procedure that you have to follow. You do not need to transfer/rollover to traditional IRA and then to ROTH. You can convert to ROTH directly from your 401k.
2. There is no limit on how much you can convert. Please note, 401k money are pretax money so when you convert them to ROTH, it will be added as income and you will pay taxes on that. So if you convert large amount, it might push you into higher tax bracket. So before you make conversion, you might want to use our tax calculate to see what your tax liability will be.
https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?srsltid=AfmBOop4s_6YPmeVKo13W7ai8MpxxvX...
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