I made these quite awhile back but I wanted to share my experience and am finally making myself sit down and do it.
I used a mix of two recipes that I found online. I read through the Rookie mag DIY and the Martha Stewart description and found a happy medium between the two. What I picked up from both that I found really important is that you do not want to put too much of the liquid ingredients into the dry too quickly or it will fizz away. You don't want all of the fizz to go away before you get a chance to stick them in the bath. I used most of the MS recipe but I didn't put sugar in mine instead I used sea salt. It's much better for your skin than soaking in a tub of sugar. I did use the corn starch because it helps keep them together better when you're taking them out of the molds.
I found my rubber molds at the Dollar Store and they worked great. I also recently found some skull rubber molds and I can't wait to use them to make black skull bath bombs.
I would suggest putting more essential oils in if you can without ruining the consistency. I used peppermint essential oil and used a small amount of coconut oil as well. My opinion is as long as you keep a damp sand like consistency then you can mix whatever you want into it.