In terms of the best type of wood to use for the frame, feel free to choose lumber based on aesthetics rather than strength. Interior doors and frames are not exposed to the same harsh elements as exterior ones, so your main concern should be picking a type of wood that you like and which goes well with the door you’re planning to hang on it. Common wood types used for interior frames are: Alder Fir Birch Pine (the most popular)
Measure your door carefully and cut the opening to fit using a reciprocating saw. Make the opening the width of the door plus 2 inches (5. 1 cm), and if additional studs are needed, a space for these, as well.
To make the studs, measure the height of the door to be installed. Cut 2x4s to the height of the door plus 1 1/2 inches (5. 1 cm) to allow room for the top of the frame and allowance for leveling the jamb. To make the door header, cut 2x4s to the width of the original opening. A “king stud” is one that runs continuously from the top plate (usually a double plate) to the bottom plate. The “jack stud” is nailed to the king stud, but it’s shorter as it supports the door header.
You will need to add 1/2" of plywood between the 2x4’s to get the right wall thickness of 3-1/2". This is the actual thickness of a 2x4.
Don’t nail the sole plate to the floor between the jack stud locations because this portion of the plate will be removed before the door is installed. Use Tapcon screws (or other appropriate fastener) to insert the sole plate.
