You can get the berry juice by going to the grocery store and picking up some frozen strawberries. Just thaw them out and pour them into a straining bag in a bowl. If you can hang the bag and let it sit in the refridgerator for a day and drip into the bowl that is even better. (Don't squeeze the bag) Do this and you should get a nice strawberry syrup to backsweeten with.
I also agree with Wade. When using the "sweet reserve" method to backsweeten, I like to do the sweetening before it clears. You don't have to do it this way though, just be warned that if you do backsweeten after clearing you may need to let it settle again.
Oh, if you aren't using campden tablets, then it would be 1/8tsp per gallon of K-Meta when adding it to your wine.