Tax Year 2016: Full-time salaried position, 2nd job independent contractor in July 2016. Have not pre-paid any taxes, believe I missed the September quarter. Where do I go from here?

I have a full-time salaried position, and recently started a second job as an independent contractor in July 2016. I haven't pre-paid any taxes based on my independent contractor work yet, and believe I missed the September quarter to make the first payment. I'd like to avoid penalties if possible. Where do I go from here?