These are made of soap and you just shave them off into your soap to get the desired color. Scent is usually added just before pouring, to try and keep as much of the scent as possible. They come in a huge variety of scents. There isn't really a set amount of FO or EO to add to your soap. It can be a matter of preference.
Some people like to use about a tablespoon of oil per pound of soap.
One thing to keep in mind though, some fo's and eo's can change the color of your soap. I don't have an actual list yet, but hopefully soon we can add this as we get it. For example though, I used a herbal fo, and it had a slight green color to it, and it changed the clear soap to a slight green hue. It was still transparent. Then again, the peppermint eo I use does not change it at all.