Do a search here on this subject.
There is at least one entire thread devoted to this subject.
I've done this as well!
I took a small half-ripe apple off of the tree in my backyard.
I sanitized a pair of needle-nosed pliers, grabbed the apple's stem and pulled it off of the tree, and placed it untouched, and uncontaminated by human hands, into a pint of sugared water mixture in a quart Mason jar, with an airlock fitted into a drilled and rubber grommeted lid.
Once I saw yeast activity, after about a week (I should have kept notes) I removed and discarded the apple, added a pinch of nutrient and about a half an inch of apple juice to the jar.
Once the culture is going good, another day or so, it goes into in the fridge until needed.
Add a teaspoon of sugar every few weeks or so, so that the mostly dormant yeast cells will have food till they are needed again.
This method will allow you to always have hydrated yeast ready to go. When you draw a portion out to use, replace the volume with apple juice, and leave it out of the fridge for a couple of days so they can reproduce (re-populate themselves) and be ready to go again, the next time to need them.
Pogo
Edit - The thread that I'm refering to is on the second page of this forum by jay415 and titled 'Quick sweet cider for the impatient'!