Ask HN: How does one go about opening a company similar to Betterment?

Can anyone point me to the specifics involved in opening a company similar to Betterment, in that it manages a portfolio and trades ETFs on behalf of their customers?

I know this would involve opening an investment company, which would lead to dealing with the SEC and FINRA, but I can't find out how the actual trading of ETFs would take place. I figure I would need to find a brokerage with very low trading fees, but does that even exist? Betterment seems to have its own brokerage (Betterment Securities), but that's a whole new level of complexity in starting the company.