1.Use offset printing or silk-screen printing to print the words or pattern you need, use any colored ink you prefer, please try to avoid very dark color, used the color as light as possibl. 

2. Mixed with water react pigment powder with water based ink, and the ink need transparent, the ratio we recommend 20-35% water react pigment. 
Or you can make the water react pigment ink directly, recommendation as Hydrochromic powder: 30%  Resin: 40%  Around Water: 30%  Thickening agent: 0.1%  Emulsifying agent: 0.1%  Preservative: 0.1%  Dispersing agent: 0.2%

3. After the first time printed ink totally dry, use 43-77T silk-screen printing net to print water react pigment ink on pervious words or pattern. 
After it dry, it will become a white film and cover pervious words or pattern, when it wet, it will be transparent and show bottom words or patterns. 
If you prefer to add covering, you can print it again. 

4. Wait to day by natural or use hot air of 80-100 ℃ for 3-5 mintues.