Creating a custom segue in xcode

Creating a custom segue in xcode

I am trying to use a custom segue in xcode storyboard to have a view slide
back down (the reverse of cover vertical).
I have learned that i need to call:
[self dismissViewControllerAnimated:YES completion:nil];
But where and how do i do this? Should this be in the .h file? And will
this then create a transition i can then choose from the drop down menu
when selecting a transition?
I am new to this so think i need a step by step guide. Thanks in advance!