how to write the code to control a CRT monitor?

Do you want to support different resolutions or one fixed resolution?

Assuming you have a RGB HS VS Monitor.

The way it works is that you need 2 counters, one for the Vertical direction and one for the horizontal direction.

On every output clock cycle the Horizontal counter is incremented by one.

The vertical counter is incremented when the horizontal counter has reached the maximum line length (including syncs, back- and frontporch) and then HCOunter is resetted.

You need then a state machine wich generates the sync signals at the correct time in the horizontal region and the statemachine has to enable the RGB Output when the HCounter and the Vcoutner are in the active pciture region. At the end of the last line you need to reset both counter HCounter and Vcoutner. Thats it in short.

Regards Christian