• Rubber-band 5 or 6 dried out water-based markers of a similar color together.
• Pour water into glass baby food jars below the screw top area.
• Put the color tips of the markers into the water and just let it be. The longer it sits the better.

you could also break open the marker and soak the markers reservoir (the part that holds the ink)
just be careful when handling as it will stain your fingers

If you use alcohol it makes alcohol ink. Permanent ink. Will stain anything it comes in contact with.

You could also cut the markers reservoir into 1/2 inch section and leave them in the jar. Even when the water or alcohol evaporates you can add more water or alcohol and revive it in the exact color every time.