In most cases configuring is automatically taken care of and you have nothing to do. Linux should detect your serial ports, and load driver modules if needed. Then the driver should insure that IRQ and address space resources have been allocated. Finally, the application program which uses the serial port should set the port speed, etc.
For any of this to work, serial support must either be compiled into the kernel (by you or by whoever compiled your kernel) or provided by modules that are loaded into the kernel when you start to use the serial port. In most cases, if it hasn't been compiled into the kernel, a module(s) will do the job and Linux should hopefully automatically find and load the correct modules.
But if you have more than 4 (or perhaps 2) serial ports, then the kernel must be told this as it doesn't seem to do it automatically. See Number of Serial Ports Supported, Kernel Configuration and. Serial Modules.
Once the proper support is in your kernel and modules, then The rest of the configuring of the serial port should happen automatically. This is done by the serial driver software often with help from your application software. But sometimes it doesn't get configured right and then you need to do it yourself. Or perhaps you need to configure it in a special way, etc. This HOWTO only covers configuration of the serial port itself and not the configuring of any devices attached to the port (such as a modem or printer).
Resource allocation (locating the hardware or low-level configuring) is assigning each port an IO address, IRQ, and name (such as ttyS2). This IO-IRQ pair must be set in both the hardware and told to the serial. We might just call this "io-irq" configuring for short. The "setserial" program is sometimes used to tell the driver. PnP methods, jumpers, etc, are used to set the IO and IRQ in the hardware. Details will be supplied later. If you need to configure but don't understand certain details it's easy to get into trouble. See Locating the Serial Port: IO address IRQs What is Setserial
The second part (high-level configuring) is assigning it a speed (such
as 115.2k bits/sec), selecting flow control, etc. This is often
initiated by communication programs such as wvdial, PPP, minicom,
picocom or by getty (which you may run on the port so that others may
log into your computer from an old-fashioned dumb terminal connected
to the port). However you will need to tell these programs what speed
you want, etc. by using a menu or a configuration file. This
high-level configuring may also be done manually with the stty
program. stty is also useful to view the current status if
you're having problems. See the section
Stty