You send the command word/ data byte just as you would if you were using the parallel port, the computer understands how the serial port works and for the want of a better description.. it turns the data sideways.
The ever usefull (and free) quickbasic software looks something like this.
OPEN "com1:9600,n,8,1,,,,RS" FOR OUTPUT AS #1 [this opens the serial port for output]
OUTPUT #1, motorcommad [ sends the data to the port]
Sadly there is no OPEN COM statement in Visual Basic and instead you need to use the MSComm ActiveX control to do serial I/O or write more complex code using the Windows API.