All mixing and stirring of components should be done before any surface application.

Catalysation of polyester can be rather difficult, considering the small percentage (1-2%) of hardener required to start the cure.

However, epoxy, depending upon which brand you buy, is usually combined at a much higher percentage, so it's easier to get the mix done more accurately.

Pick up a small postage scale, something that will show you a gram or ounce readout, and then you'll be able to combine them at exactly the percentage required. I use the West system epoxy, which mixes at a relatively easy to measure, 5:1 ratio.