How do I setup Time Machine without losing my previous Time Machine backups?

[wannabelean]

I just installed Yosemite on my 2012 13" base model MBA and have a clean/plain vanilla system for me. Now when I connect my time machine backup hdd to the system it asks me to set it up so that time machine can backup my system. I kinda concerned and worried that if I set it up wrong I might end up deleting my existing time machine backups. How do I setup time machine backup and yet making sure that it does not touch/edit/delete any of my existing backups?

 

[Weaselboy]

The safest way would be to shrink the existing partition on the external drive then add a new partition at the bottom and point the TM backup to the new partition. This way you can be sure the new backup won't touch the old one.

This will help you with the partitions.