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
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.