I started giving plain rice cereal to my baby when he was 5.5 months. The brand of the rice cereal I use is "Earth's Best" - you can buy it from citysuper, large park N shop and welcome. As I still am breast feeding, I mixed it with breast milk. But I know it is ok to mix it with formula milk (just prepare the formula milk in the normal way and use however much you need to mix the cereal into the right consistency). When I first started feeding rice cereal to baby, I use 1tablespoon of cereal mix with approximately 1.5 tablespoon of milk, and I only feed baby with 3-4 teaspoons to let him taste first(and not as a substitute for his milk feed). Once you see that baby does not have negative reaction to it, you can gradually increase the amount of cereal and also make it thicker (i.e. add less milk). To give you an idea, my baby now takes 4 tablespoon of cereal mixed with 5 tablespoon of milk and 1 tablespoon of vegetable puree.
After seeing that my baby is ok with plain rice cereal for a few weeks, I started preparing vegetable puree (using blender and I buy organic vegetable if I can) and add it to rice cereal to feed baby (again starting with 2 teaspoons of vegetable, and gradually increasing the amount if baby is ok). whenever you try a new kind of food for baby, make sure you only give one new kind of food (that way you will know for sure which food is giving him a problem) and test it for 3-5 continuous days to see if baby is allergic. If you would like to know more, please PM me.