An internal modem is easy to install. The only considerations for installation are that you have an available slot and that the modem is designed for the slot type you have. Most PCs use a PCI or PCIe slot for a modem. Older PCs used an ISA slot; which is now obsolete. Check that you have an available slot and the slot type, before purchasing a modem.
Before installing the modem, unplug the PC to avoid an electrical shock hazard. Exercise caution as there are sharp edges on a PC and its components. Always wear a static electricity discharge strap when working with your PC's components to avoid damaging your PC. Always follow the manufacturer's instructions that were supplied with your modem.
Remove the cover plate adjacent to the slot for your modem
Align the modem over the slot with the phone jack facing toward the outside of the PC
Set the modem into the slot and press down firmly. Make sure the mounting bracket is properly aligned with any slot that might be present. It may be necessary to gently rock the modem from end to end to secure it in the slot. The gold contacts at the bottom of the modem should nearly or completely disappear into the slot. The card should be level and straight.
Secure the modem mounting bracket to the case with a screw (or replace the securing mechanism your case uses.
Replace the cover
Plug in the PC
Plug in the telephone wire to the modem jack labeled "Line" and plug the other end into the telephone outlet on the wall.
You may also plug a telephone into the modem jack labeled "Phone"
That completes the hardware installation, next you will need to start the computer and complete the software installation. Your modem should have come with a disk which contains the appropriate drivers.
Follow the manufacturer's instructions for installing the software. Windows will detect the modem and start the hardware wizard; simply follow the prompts. Some manufacturers require you to cancel the hardware wizard and complete the installation using their own installation instructions.