When do I start harvesting my red kidney beans?

I assume you mean you are growing some kind of red seeded haricot? If so, you just leave the plants to grow, flower and pod up. Wait right to the end of the season until the pods are turning dry when it will be easier to shell them. Dry the beans off completely before you put them in glass jars for storage. Check the jars frequently to make sure you haven't got bean weevil, which is a real pest here. We sometimes freeze any which are not completely ripe instead as they don't take up much space and are quicker to cook without soaking for so long as dried beans.
If the autumn is wet you may need to pick them early to avoid them rotting in the pods. The French pull out the whole plant and hang them in a barn to dry out completely and then thresh the plants and pods over a sheet into which the beans fall.
In our experience though, you need an awful lot of bean plants to get even a relatively small number of dried beans.