The three recipes here all tell you how to dissolve Indigo, and how to dye with it. Deeper history and chemistry may be found in any good dye book. They should be treated with care and common sense, but without panic. Keep them dry, out of children's reach, away from food and use clean dry utensils for measuring. Wear rubber gloves, don't splash, especially keep out of eyes.
Water Put the water in a glass jar that has a close-fitting lid.
Slowly add the Sodium Hydroxide while stirring. The Solution may get quite hot. This is a strong alkali and should be handled very carefully. Close and label the jar. This can be kept indefinitely, but should be clearly marked.