How do I make a homemade skeleton key?

That depends on the lock. Contrary to popular belief, a skeleton key does not open every lock. A skeleton key is a type of key used in old locks. If you have an item which requires one, find a key blank that fits the lock. Next, either use a black sharpie or hold it above a flame to blacken the key. Put the key in the lock and try turning it. Pull it out and notice the shiny spots. File them a bit, then blacken the key again. The general idea is, when the key stops having shiny spots, the lock will open. This is a form of "impressioning."