I bought my opk from the supermarket for about $20. You get five tests in a pack. They have instructions in the box which tells you how to work out when to use them. You start testing about 3 days before you ovulate so you don't miss the rise in LH which is the hormone which makes the ovary release an egg. The kit tests for this hormone. Sometimes you need to test for longer than 5 days and sometimes you may not ovulate in a cycle so it can be expensive. Hope this helps a bit