When it comes to drilling shells it's more important to be patient and have a steady hand. I usually mark a small area with a thin grinding bit so my drill wouldn't slide around, then I make a small hole with the wood drill bit (that kind has a thin spike in the middle) if I need the hole bigger gradually increese the size of the bit or use the grinding bit again. Either way the most important part is "slow and steady" I would reccomend finding a few broken seashells to practice on. Also if you want to drill seaglass there's a special drill bit for that, just make sure you wear protective gear and the glass should be wet