Yeah its just a case of getting a bottle of beer with some of the orginal brewery yeast in the bottom (beware some producers filter the orginal yeast and bottle with a more stably sedimenting fresh yeast).
Pour out the beer and (drink it
) leaving about an inch in the bottom with the yeast, then either flame the mouth of the bottle or wipe with betadine/sterilising solution and then transfer into some fresh wort (10 g of spraymalt in 100 mls of water) which has been boiled (to kill nasties) and chilled.
Grow it up in the flask for a few days (give it a good swirl about every now and then to help growth) after a few days step it up to a litre of wort and again after a few more days allow the yeast to sediment out, decant of most of the exhausted liquid wort and then pitch all those yeastie into the full size 5 gallon batch.
Probably a good idea to keep a very close eye for possible infections (you don't want to ruin a 5 gallon batch just to save a few quid).
SW- I don't think a lagering peroid is required (i might be wrong though) just a reasonably decent peroid of conditioning, about two months im guessing.
