framing an opening

I have moved into a house that has a squared opening (about 8' wide) going from the family room into the dining room. It kind of looks like a squared arch, but is only drywalled with no trim.. I would like to frame and trim it. I assume that it would be relatively easy. It is 5.5" thick.

I assume that all I need to do is install 1 x 6 along the "jamb" portion of the opening and then install casing along the outer edges on each side. Of course, I would mitre cut all the angles. Am I oversimplifying this? I have looked for specific instructions on how to do this but can't find anything anywhere. Any help would be appreciated.

 

Huck-- sounds like you,ve got it figured right. Instead of mitering the corners, you can get square rosettes for top corners and make square cuts on casing. Only prooblem I see is that 1x6 lumber is only 5 1/4 wide--may have to get 1x8's and rip them down--or maybe you can find 5 1/2 interior jambs. Good luck Chuck

 

Huck344- Also, run your top jamb across full width and set side jambs underneath---Chuck

 

Thanks for your help and confirmation. Hopefully, I won't screw anything up.