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Hi Vicki,
Here's a tutorial on how to frame a doorway, you just might have to adjust it a bit. Wishing you the best.
https://homeguides.sfgate.com/frame-30inch-door-27901.html
With shallow walls, use 2x2 instead of 2x4 lumber. Just know that it will never be as strong as a regular 2x4 wall
I'm assuming it's a non load bearing wall, so take a look:
https://www.finehomebuilding.com/2016/09/30/frame-door-opening
https://www.youtube.com/watch?v=m4RSgE-h2Mc
Is it a load bearing wall?
“flat stud” the framing, as in turn the 2x4’s sideways. This includes the header.
There’s more math here than the framing:
1.) Ripping down the jamb sets.
This dimension is the framing @1.5” + drywall @ .5” (x2) = 2.5”. This is if you’re in the US.
2.) The door is 1.5” thick. The stop is 1” thick. That’s right on your rip-line.
To frame the doorway you would need a saw to trim off normal board sizes and cut them length wise to get the fit. Other than that, it seems it could be done like any doorway frame. Also, talk to your local home improvement store people. They love to help and advise. Make them a friend. Ours ask us how certain projects went after we asked them for help. They are great people!
If you're using a standard size/thickness door then the door jam will not be flush with the wall as in typical construction. Lifestyle Homes has given you some great tips above just know that your door will stick out on both sides of the wall, unless you choose to make it flush on one side and not the other, but then you won't have the best support. You also need to make sure the header above the door is sufficient to transfer the load of the wall around the door.