Upgrading Domain Controllers from 2008r2 to 2012r2

I would like to upgrade my two 2008 R2 Domain Controllers to 2012 R2, and I would like to know any special steps or considerations I should take.

Environment Specifics
•      There are two DC’s, both running 2008 R2
•      My Domain and Forest functional level are 2008 R2
•      Both servers are GC and DNS servers
•      One of them has all the FSMO roles
o      It also is our sole DHCP server
o      It also is our only NTP server
•      I don’t do anything with Certificate Services or WSUS

Specific Questions
•      Should I go through 2012 first?
•      Should I upgrade the one that doesn’t have the FSMO roles first?
•      What kind of downtime should I plan for?

Thank you in advance,

Mike