you are cutting in a pass-through.Chances are you won't be going stud to stud.
On the sides,cut your studs your R.O. and screw through the drywall into the stud to hold it.The header you want to use channel.Cut 3"longer on BOTH ends.Cutthe sides of the channel 3"in on both sides and bend down.This will give you an "ear" to screw to your studs.You will have to work the channel up between the drywall and the studs you cut then screw through drywall to hold then screw the "ears" to your side studs.
An air nailer is the way to go.A bit of glue helps.Trim screws are an other way but costly and take time.
If you are hanging doors in the opening,I would cut the width 3" wider and the heighth 1 1/2"taller and wrap with 2x4 screwed to the metal.
Use screws that are fine thread.Self tappers are nice but not needed(I'm guessing you are working with.25guage steel)
I wish I could be there to show you because it's hard for me to put into words what I know(exept my opionions)
Post back with ???Luck