transfer a keychain

I had to reformat my HD of my Mac Pro. When I had everything set up, my keychain (history, pw, bookmarks etc) didn't transfer.

I know there is a way to do this. What is the procedure for adding a username and pw for bringing keychain info off my cloud to another computer.

I have a mac pro, mac book pro, iPhone and iPad, so I have everything stored in keychain. I was also able to do this once before. I just can't remember how to do it.

The reformatted drive is OSX yosemite.