First, you need to buy vegetable glycerin, you can get blocks of it at craft stores (the local health food/co-op here has some, its not hard to find), it's pretty cheap, you can get enough to make a good-sized bar for a dollar or so.

Then you get food coloring, essential oils or soap scents, and whatever herbs you want to put in it. Make sure that any herbs or flowers you use are dried first, otherwise they'll get weird in the soap.

Get a pan, stick the glycerin in it, and melt it on medium/low. Don't let it bubble! That's bad! As soon as all the soap is melted, add in the food coloring and the scents. Mix it around a bunch.

Then you get molds. You can buy fancy stuff at craft stores, or just use small tupperware containers. You don't want to use anything that'll melt when it gets hot. Stick in the soap, and then add the herbs you want. You might want to wait a little to put the herbs in, and then mix it around really well, because herbs tend to float to the top.