I bought them through the EOS distribution, so it's already on an (private) ethereum account as well as registered to an EOS key, something you need to be sure to do before it goes live. I had the ease of doing this all through the EOS.io site, as it has all these functions build in.

One thing you need to be sure of is that the Ethereum account you use is ERC20 compatible, so that it is able to store the EOS. That same ETH account needs to be linked to an EOS key, so that once it goes live your EOS "tokens" will get transferred inside the system. Myetherwallet or metmask are the ones usually recommended.

You could check this article for some more info (from step 3 onwards): https://steemit.com/eos/@sandwich/contributing-to-eos-token-sale-with-myetherwallet-and-contract-inner-workings