I think the easiest way to do it is to chop up the cabbage - depending on how much indicator you are making you will probably only need a quarter - and boil in water until it is soft.
Discard the cabbage and keep the now purple liquid. Once cool you have your indicator - one small tip red cabbage juice smells and it gets worse the longer you keep it!
BTW I know that many high schools and chemists extract the indicator using solvents but I really think it works just as well using water and boiling up plus primary schools often won't have access to chemicals but use whatever method you want to.
If you are making indicator paper I have found the best to use is cheap watercolour paper in a pad. You could use coffee filter paper or even school filters but art paper is by far the cheapest and available in large sheets.
Use a large tray and immerse the paper then let it soak up the colour. Let it dry and cut into strips - there you have it.
You can use larger pieces for pH painting where you dip either cheap paintbrushes or cotton buds into solutions of different pH such as clear vinegar, bicarbonate of soda and then when you paint you will get different colours depending on the pH of the liquid.
You will get reds and pinks in acids, purple in neutral and greens and blues in alkali.
If you don't want the smell of cabbage or you want to do this is summer then you can use pretty much any plant that has a purple colour - squish them up and use the juice. Blackberries work well.