Architrave is the trim around a door which over time can become damaged with items being banged against it, especially children’s toys or when moving furniture. The architrave may be a decorative feature (such as used in period decor), or you simply wish to replace a plain architrave with a more decorative design.
You could always pay a contractor to do this work for you. Even in that case it is always better to know what they should be doing, after all they are spending your money.
Measure up - Measure the internal width of the door frame and add 12mm to the overall length, this will allow the architrave to be set back from the inside of the frame.
Finish up - Fit the side lengths by butting them up to the top length and making sure that they too are set back from the edge of the door frame by 6mm. Punch all the nail heads below the surface and fill the holes and any gaps between the mitred ends with wood filler, sand down and either paint or varnish.