You can begin by making a starter with the one pack of yeast you have to get a beer going before you need to harvest the current fermenting beer. Make the starter larger than necessary so you can begin another starter as soon as the first one is done. This is the starter/pitch rate calculator I use which has a built in over build calculation. the over build gives you fresh yeast to use for each ferment.
Here are two previous threads on harvesting yeast for more information.
Hope this helps. Any question is a good question on this forum.
