Home made yogurt lacks some preservatives, so it's not really recommended to keep it for more than 3-4 days.

Making your own is actually not hard at all. You just need regular milk. Boil it and then let it cool a bit. Not exactly room temp., but slightly warmer. Then beat some yogurt (preferrably the home made one you have) and mix it in the cooled down boiled milk. General rule of thumb is one full spoon of yogurt for every liter of milk. Mix it up a bit, but gently and place it somewhere warm. Covering it in blankets next to the radiator will work great. It needs 4-5-6 hours to thicken.