First remove the cover

You must shut your computer down, and unplug the power cable, and any other cables that are plugged into the machine. To remove the cover, usually you will need to remove several screws that hold the cover in place, and the cover will then slide forward or backward from the chassis. This can vary greatly though, with some newer machines, so consult the manual or manufacture's technical support for detailed instructions.

Find an open Expansion Slot
Now you need to determine what kind of slot you have opened. You will likely a PCI card slot open, which you can identify by its short length. As opposed to an ISA card, this will be significantly longer.

Now make sure you have an open slot appropriate for the type of card you have. ISA slots will be longer, and black (see image 1 below), whereas PCI slot will be shorter and white

Removing the slot Cover

On the rear of the case, directly above, and to the left for PCI, or above and to the right for ISA, of the slot, there will be a removable cover. This may be removed either with a single screw (if this is the case, be sure to save the screw), or may need to be broken away with a screw driver.(see image below)

Insert the card

Now insert the NIC into the slot you have chosen, by placing the end of the card with, what looks like a large telephone outlet, facing out of the space created when you removed the slot cover. Press the card into the slot firmly, but gently, making sure the card is seated completely into the slot.

Last

Put the screw back into the new NIC and replace the cover and cords.