Do I disconnect negative cable under hood or can i connect battery tender under hood or what is best to do

 

You can do either. If you are storing for a very long period of time and want it to start right up, the battery tender might be the way to go. A battery will self-discharge even if not connected.
I generally drive my convertible about once a month or so (on a dry day) in the winter and it's fine. I don't disconnect the battery and it generally starts right up although if it's closer to two months, it may need a charge.