MEASURE TWICE CUT ONCE, BUT DON'T MEASURE AT ALL IF YOU DON'T HAVE TO!!!!!
My husband and I do things differently. He is a math whiz and for him adding up all the numbers in his head just adds to the fun. (36 inches for the size of the door, + the reveal 1/8 inch x 2 + 2.5 inches for the casing oh but times that by two and add all that together. Then find the middle of the door, then find the middle of the casing, blah blah blah) For me, not so much. That's two minutes of my life I can never get back and that's if I do it right the first time which I won't. So this is how I do it.
First install header- cut one end of the header on a 45 degree angle like this.
Leave the other side longer than you need it, hold it in place and mark where you want the cut. Cut at a 45 degree angle.
Nail in the header (Door molding typically sits back 1/8 inch from the edge of the doorjamb. This "reveal" leaves enough room for the hinge barrel. This measurement may have to be fudged a little depending on how your drywall was installed just try to make it uniform all the way around the door.)
Miter one end of each of the legs make sure to leave the legs longer than needed.
Hold a leg in place but upside down.
Mark where the top of the header meets the leg and cut the leg straight at the mark
flip over and install, add wood glue to miters
fill gaps and nail holes with wood putty then caulk side seams.
This is really quite simple, my 13 year old daughter (pictured above) and I installed several. I think all 13 year old girls should know how to use a nail gun.
We used a miter saw to make the cuts but this could be done with a miter box, we also used a nail gun but you could use a hammer and nails, just be careful not to split wood.