The paint(3+ months old) in my can has separated but I'm not sure what kind of tool to use to stir it. It is latex paint used for indoor purposes. Does anyone has any recommendation on what tool to use for the stirring?
I would suggest a paint plunger
(source: rokset.com.au) which doesn't require a drill and is more thorough in stirring the paint than a paint stick.
Just use a plunging motion to get the paint from the bottom of the can to mix thoroughly though the tin, they come in different sizes as you see for different size paint cans.
You can get a mixer tool that attaches to a power drill. This does a much better and faster job than the wooden paint stirring sticks you can pick up at any paint store.