After the plaster wall is cut out you can start placing the inner door frame into place. Cut them to size and drill starting with a 10mm shallow hole and another 3mm pilot hole. I used 90x20mm timber for the frame. Later on all holes will be closed with hard wood plugs.
It is very important to make sure the inner frame is square on the 45deg corners.
The best and most aqurate way to get the right door size is to make a drawing of the door opening. Use the frame material size adding that into the drawing leaving 2-3mm between the 2 doors and the frame. After the drawing is finished all you have to do is place the timber on the drawing and bring all the lines from the drawing on the timber and cut. I used planed 2x2 for the doors and the inner panel thickness is 18mm.
After all the frame parts are cut use a router to make grooves for the inner wood panel, you can also add moulding with an electric router or just glue ready made moulding when the doors are finished. After the glue is dry give the doors a light sanding.
No need for special joints just use a wood adhesive and screws into every joint closing the screw holes with wood plugs aswell.
I used my 45deg steel angles for the corners and a ratchet strap around the frame to secure it. While the glue is drying place a small mdf block under the strap to protect the soft wood.
You can add clamps to secure the frame to a flat surface that will prevent a twist.