Mix the water and lemon juice with the spoon. Dip the cotton bud into the mixture and write a message onto the white paper. Wait for the juice to dry so it becomes completely invisible. When you are ready to read your secret message or show it to someone else, heat the paper by holding it close to a light bulb.
This is a method online to make the invisible ink easily. Many people know about it some even try it. It seems to be a good way to make the invisible ink at home. But these invisible ink isn't suitable for marking poker playing cards at all.
It's of risk if to use these ink in poker games or magic shows. For safe and better pleasure, I suggest to take invisible ink marked cards
instead of doing such invisible ink for poker.