Here's how to make pH paper using the red cabbage solution and lab filter paper:
As shown in Figure 2 below, soak 5 sheets of filter paper in the red cabbage solution for about 30 minutes
Drain the excess solution from the sheets of filter paper, and set them out in a single layer on some paper towels to dry overnight. To speed up the drying process, you can put them on a cookie sheet and put them in your oven at low temperature (150-200°F).
When the sheets of filter paper are dry, cut them into strips (for example, 2.5 cm x 11 cm).
The strips are now ready to test the pH of various solutions. They start out blue, but will turn green in basic solutions and red in acidic solutions.