As a tax professional, I found this estimated tax payment calculator tool (link below) fairly straightforward to use.
TAX TIP: If the tool I linked to below seems overwhelming or confusing to you, please consider trying QuickBooks software to SIMPLIFY calculating and paying Estimated Taxes (New Improved QuickBooks Online version has a FREE 30 day trial — CLICK HERE to try FREE 30 days)
JULY 2017 UPDATE – QUICKBOOKS ONLINE now offers a SELF EMPLOYED version that will calculate your Schedule C for you!
Click on the link above to get a Free 30 day trial of QuickBooks Self-Employed. QuickBooks Self-Employed is designed for employed, &/or small business owners to track self-employment income, expenses, and estimated tax payments.
Or, try calculating your own Estimated Tax yourself w/this 1040-es calculator tool: http://www.ipolyi.com/ES.html