Installing doors can be very complicated if you haven't been well trained before embarking on your project. Benefiting from some good advice and knowing which tools to use will allow you to carry out your project successfully.
Steps
Part 1 of 4: remove the old door

Step 1. Peel off the trim around the door
Do this gently by slipping the end of a wood chisel behind the trim and sliding it all the way around the door frame. This way the paint will get damaged and come off less when you remove the trim.

Step 2. Completely remove the trim surrounding the door
You can do this carefully with a hammer and a wood chisel. Insert your chisel between the wall and the casing then strike it lightly with the hammer. Finally, move the chisel back and forth to detach the cover. Set the cover aside, whether to put it back on later or to throw it away.
Take special care when removing the trim if you intend to reuse it after your door has been installed. Damaged or broken casing will give your door a less professional look once installation is complete

Step 3. Remove all nails and screws that secure the door frame to the wall
A saw, wood chisel, and hammer may be needed to completely dislodge nails and screws.

Step 4. Once the nails and screws have been removed, close the door and swing it over to the side where the trim has already been removed
The door should pass easily through the opening. It is recommended to have a friend or relative by your side to help you, especially if the door and frame are heavy.
Part 2 of 4: prepare the premises and the door

Step 1. Start by checking your opening
This is to measure the width and height between the edges of the rough opening. The rough opening should be 5cm wider and 5.7cm higher than the new door you are going to install. If the opening is too big or too small, you will have to correct it, otherwise the installation will be very complicated.
For example, if you are installing a 30 '' wide door, you will need a 32 '' wide opening. For a door of normal dimensions, 200 cm high, you will need an opening of about 209 cm high

Step 2. Prepare the door for installation
Remove any wrapping or fasteners still on the door. The guards on the sides of the door must also be removed. Remove the handle from the door; if you don't do it now, you will have great difficulty doing it later.
Part 3 of 4: install the new door using brackets

Step 1. Use an effective square or leg hanging system for the doors
The brackets allow the doors to be hung very satisfactorily. They are easy to install alone and will cut your installation time in half. The brackets will also allow you to adjust the position of the door more easily if necessary.

Step 2. Using a level of 1, 2 or 1.8 m, draw a vertical level line on the wall 1.3 cm along the opening
This line should be on the side of the door hinges and in the room in which the door opens.
Make sure the line is straight and plumb. A straight line that is not plumb with the ground will not allow your door to swing properly

Step 3. Screw three brackets inside the door frame, next to the hinges
Using about 1.3 cm screws, install the brackets next to each hinge making sure they are equidistant from the edge of the door frame.
It is very important to position the brackets just at the level of the hinges. The hinges are the most important place of tension on the door. Thus, improperly installed brackets, misaligned with the hinges can lead to an error in the axis of the door

Step 4. Screw the other three brackets on the opposite side of the door frame
Start 8 inches from the top, then 2.5cm above the handle and finally 15-20cm above the ground. It is recommended that you place a last square, if you have one left, at the top of the door if it is 76 cm or more wide. The door is now ready to be inserted into the opening in the wall.

Step 5. Place 1.5cm wedges on the floor for carpet and 0.5cm wedges for laminate floors
Never install a door directly on the floor.

Step 6. Starting with the top hinges, place the brackets on the reference line drawn on the wall at the beginning

Step 7. Screw the door to the wall using the screws provided
Work along the side of the hinges, starting at the top, making sure to use the same reference line for each of the three brackets.

Step 8. Next, check the gap between the frame and the door at the top of the door and the handle side
Make sure it is constant. It is easier to screw in the fasteners now than later, when the last screws are lodged in the wall.

Step 9. Starting again at the top, continue to screw the hinges on the side of the handle
Keep the distance between the door and the frame constant as you screw the door to the wall.

Step 10. Secure the trim over the brackets
Most covers have a hollow back, which allows the brackets to be placed and hidden. If your cabinet does not have a hollow back, you may need to hammer the brackets lightly into the wall.
Part 4 of 4: use dunnage plates

Step 1. Place three wedge plates at each hinge on the inside of the door

Step 2. Drill a pilot hole through the wedge plate and hammer a nail into it to hold it in place on the door

Step 3. Using a 1.8m level, check that the door is straight and plumb
This is to use the level to ensure that the positioning is straight and plumb. If not, you will need to adjust the plates to straighten things out.
- Place the level along the frame, hinge side, to check that it is straight. If not, adjust the top and bottom plates until the level indicates the frame is plumb.
- Once this is done, place the level on the frame to check that it is plumb. If not, adjust the frame and plates until the level indicates the frame is plumb.

Step 4. Drill pilot holes through the middle and bottom wedge plates
Drill nails into the holes and secure the frame to the wall.

Step 5. Line up three more wedge plates on the handle side, perfectly opposite the plates on the hinge side
Make sure they are flush with the faceplates.

Step 6. Drill pilot holes through the top, middle, and bottom wedge plates on the handle side
Finish the operations by hammering nails into the pilot holes, always checking that the frame is still straight and plumb.

Step 7. Place two plates on top of the door frame
Drill pilot holes through the wedge plates at the top of the door frame and secure each with a nail. Check that the door and its frame are evenly spaced.

Step 8. Pre-drill eight pilot holes at the wedge plates on the short side of the frame
After drilling, drive eight nails into each hole and secure the frame to the wall.

Step 9. Fully drive the nail heads into the door frame or using a nail set
This will prevent the nail heads from obstructing the door when it swings.

Step 10. Replace the screws on the hinges with screws of about 6 cm in order to provide more stability and balance

Step 11. Break off all ends of the wedge plates protruding from the door frame
Do this either using a hammer or with your bare hand. File and sand any small, sharp ends that are difficult to access with a larger tool.

Step 12. Install your skin on the frame
Reuse used trim around the old door or install a new one to match your new door.