anyone self-employed have more details?
Yes, I have my own S-Corporation and run my own Roth 401k. I had a company called Broad Financial set it up. An employer (in this case my corporation) has to offer the 401k, so no luck if your employer doesn't offer one and you aren't self-employed.
For a Roth 401k, the employee contributions are Roth and are currently limited to $17500 a year. This comes out of your paycheck, but taxes are still due on what you contribute, like any Roth contribution. I set up my payroll to record the amount transferred every month.
The employer contributions are considered traditional contributions and the total contributions, employer plus employee contributions, cannot exceed $52000. So with the maximum employee contributions of $17500, employer contributions can be at most $34500. This comes with the caveat that employer contributions can't exceed 25% of your compensation, so to contribute $34500 from the employer, you have to be paid at least $138,000 in salary.
The mechanics are pretty simple. I have two trust checking bank accounts (free through PNC) in the name of my company. Every month I take a corporate check, write it out in the name of "XXXX Trust" for the amount of my employee contribution, write "FBO [My name] Roth 401k [Month] [Year]" in the memo line and deposit it. I have a second checking account for the employer deposits.
From there I invest in whatever I feel like in the name of the trust. I had been doing prosper loans, recently moved to bitcoin. IRS reporting requirements are pretty minimal until the value exceeds $250,000. Roth contributions don't affect my liability for the current years so there's not much to report on the personal side either.