How do I invest cast into the account once it's open?

Once your account is open, the easiest way to add more cash to these accounts is to do an electronic funds transfer online.

The instructions for Fidelity.com can be found here:
https://www.futureadvisor.com/instructions/Fidelity_EFT_Instruction...

The instructions for TD Ameritrade are here:
https://www.tdameritrade.com/retail-en_us/resources/pdf/TDA693.pdf

Once our algorithm detects cash in your account it will be invested with 48 hours. There is no minimum for how much cash you can add to your account. However, we may not automatically invest cash balances under $200 as the trading commissions may be too high.