How do I calculate provisional tax and when/where do I pay it?

I am a freelance writer and my income depends solely upon jobs that I get from international clients. I get paid weekly in dollar and my income fluctuates. Recently I've started earning more and I suspect I might be in a tax bracket. As far as I know I have to pay provisional tax but how do I work it out and where and when do I pay it?

The tax threshold is currently R75,000 so if you earn above this amount in a year you should be paying tax.
Please read our blog on What is Provisional Tax? How and when? and also work through our online questions Am I a Provisional taxpayer?

Tax filing season opens 1 July 2016! Please Register for TaxTim and let us assist you to complete and submit your tax return to SARS.