FC - all you need to do is follow this write up:
https://www.homebrewtalk.com/entries/yeast-harvesting-novel-approach.html
I use it all the time and have a sweet bank of clean yeast in the fridge. Go to yeastcalc.com and run the numbers to build up something big enough that you can afford to pour off .5L for a future starter. You'll probably have to do some kentucky windage since you don't know how many cells your starter has going.
https://www.homebrewtalk.com/entries/yeast-harvesting-novel-approach.html
I use it all the time and have a sweet bank of clean yeast in the fridge. Go to yeastcalc.com and run the numbers to build up something big enough that you can afford to pour off .5L for a future starter. You'll probably have to do some kentucky windage since you don't know how many cells your starter has going.