Reformatting 2.5" HDD ready to sell

So I have upgraded the HDD in my macbook and I wish to sell my old one. How do I reformat the old hard drive so it is securely clear of all my data not its not in my macbook?

Thanks

Joe

You can reformat it in disk utility

here is a button called Security options, you can choose the level of overwrites from there, from the simple to a 35 pass overwrite, each stronger option takes longer

35 pass might be overkill - depends on how sensitive the data is

thank you for the reply. So with the old hdd in the macbook I can overwrite the drive, remove it and put the new one in?