Before December 31, 2017:
- Sign adopting resolutions and a plan document, and
- Deposit a de minimis amount, e.g., $1,000 in a trust account to establish corpus.
After December 31, 2018:
- Fund the balance of the deductible contribution no later than the time your tax return is filed including extensions, and
- File Form 5500, if applicable
And if this is your first retirement plan, check with your accountant to see if you are eligible for the tax credit for pension plan start up costs.