In this instructable I will show you how to create extra space in your home for all the things you usually stumble over scattered on the floor. This is a fun project and could be done in a day or two so lets get started.
This project doesn't require too many tools mainly a hand saw or electric saw to cut all the timber parts. If you don't have a router you can use ready made mouldings and glue them on.

Safety : When using electric tools use ears and eye protection.
Keep body parts away from sharp blades. WORK SAFE!

Step 1: Decide the Door Size and Cut

Make sure to check that there is no electric cables or pipes behind the wall, you can use a wire detector for this task.

After deciding the size of the door start cutting the plaster board with a plaster board saw. I personally don't like to use electric saws because you never know what you might chop behind a plaster wall.
Start with a small square hole that will allow you to look in and feel with your hand where the joists are and if there is any thing you dont want to cut. Use a measuring tape and mark with a pencil and draw lines on the plaster wall where it's to be cut .
After the plaster board is cut out use a timber saw to cut the joists that are in your way.
Use plaster wall nails or screws and secure the plaster board edges where you cut the opening is. If necessary add new back support joists for the plaster wall and support for the doors inner frame.
When you open the wall you would be suprised how dirty it could be behind that wall.
A bit of hoovering and we are ready to continue.

Step 2: Inner Frame and Doors

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.

Step 3: Hinges & Door Stopper

After 2 of the doors are ready test fitting, if the doors are too big don't panic use a small sharp block plane and shave a bit off the door and try fitting again.
Once the doors fit in nicely make recesses for the hinges. I like to use brass hinges with pulll out pins that allows you to remove the door at any time without taking off the hinges.
After the hinges and doors are secured in place close one door and mark where you want to nail the door stopper for both of the doors all around the inside frame. The stopper can be rounded or beveled on the edges and acts as a decorative purpose and will prevent chipping. After the stopper is in place attach your locking system. I used locking clamps they are cheap but excellent for this type of storage doors.

Step 4: Door Handles and External Door Mouldings

This is optional: attach a strip of timber to one side of the doors half way out covering the middle gap between the two doors. After the doors are in place you can attach any handle you like that will match your doors style.
I placed a few hooks inside the door and an iron holder I got in Ikea.
Last cosmetic thing to do is place decorative moulding around the door, close all the holes in the plaster wall with filler and close any remainig screw holes with hard wood plugs.
If you have a big space you can add a few shelves, a small light and a bit of painting.

Now before you know it the new storage place will be full and you will scratch your head wondering where you can make more space. .. . :-)