Cut up the red cabbage into tiny pieces. Cut up about a cup's worth of cabbage and put it in a blender, with about 1.5 cups of water. Blend it until the cabbage is well mixed with the water.
Use a fine strainer to filter out all the cabbage, leaving behind just the juices. You may have to strain it multiple times to get all the cabbage out.
Soak the coffee filters in the red cabbage juices. Let it dry. Cut pH testing strips out of the coffee filters, which now contains the flavin compound.
Use your new pH strips to test the acidity of whatever compounds you want to test. Pure red means low pH (pH 2,) purple and violet means pH 4-6, blue is pH 6, green is pH 10 and yellow is pH 12.