So... this is a laptop? Are you sure it's PCMCIA and not an ExpressCard slot? It'd probably be a fair old age if it's PCMCIA
If it'c PCMCIA, which IIRC only had one size, you'd just need to pick up a card with USB ports and slide it into the slot.
If it's ExpressCard, you have to be careful to pick out the right card size for your laptop, after that it's just a matter of sliding it in, same as above.
If it's a desktop, you need a PCI card. You open up the case, take off one of the protective sheaths from the back panel, and slide in the card into an open PCI slot. Snaps in and is very simple to do.