It's easy. Simply finish your tax return, select File in the left menu, and follow the instructions until you can select the Transmit my returns now button.
You must select that button to electronically send your return to the government.
Once you've done that, the next thing you should see is a Transmission Successful screen. This means your return is on its way to the government agency.
Your e-filed return will remain in pending status until the government either accepts or rejects it.