1. Turn the power off and take the case off of your computer.
2. Choose a slot for your modem. Most modems require an 8-bit slot, but some may require a 16-bit slot. 16-bit slots are longer than 8-bit slots; your motherboard probably has both. Try to pick a slot with room on either side to give your modem plenty of room.
3. Remove the corresponding metal plate from the back of the computer. Save the screw for use with your modem.
4. Remove the modem from its packaging. Handle it by its edges to avoid static electricity. Don't touch its connector pins or the chips on the board. Push the modem into the slot. You might need to roll the card into the slot to get a snug fit.
5. The back plate of the modem should line up with the open hole in the back of the case. If so, tighten the modem into place using your saved screw.
6. Now attach the phone lines to the modem. The line from the wall jack goes to Line on the modem. Then, if you want to, you can connect your phone to the other jack on the modem, labeled Phone.
7. Reconnect all cables and turn the computer back on. It's probably best to leave the case off until you have tested the modem, just in case you need to remove it again.
8. The computer should boot normally. If you are using Windows 95 and your modem is plug-n-play compatible, your computer should detect the new modem upon startup. In this case, just follow the instructions to install the software. Under Windows 3.1, you can first test your modem in Terminal, under Accessories. Type a command, like AT, in the box. If it responds with OK, the modem is probably working. If you can't type or you get an error, your modem is not set correctly. You need to choose a different COM Port, or IRQ. If it is working, install the software.

High Speed Considerations 

If you're planning on using a 28.8, 33.6, or faster modem, make sure you have a 16550 UART, or Universal Asynchronous Receiver/ Transmitter. This manages the communication between your modem software and the modem. Your modem may be able to operate at 115,200 bps, but with a slower UART, like 8250 or 16450, you will have data loss at these transfer speeds.

You can check your UART through MSD; many other diagnostic programs might tell you as well. Most modern computers have it. If you can't find out, just keep this in mind if you have problems later.

If you do have an older UART, slow the software down. Choose 9600bps or 19200bps. Even if you have a fast modem, your software the then limit the modem to this speed.

The good news, though, is that internal modems don't have to worry about this because they have an updated UART on the card. This overrides the one on your computer. External modems are limited my the computer's UART, though. The only way around it is to by a new I/O card.

Configuration

Making an internal modem work can be a real nightmare because of IRQ and COM Port conflicts. Your modem cannot share an IRQ or a COM port with another device.

To change your modem settings, you'll need your manual. Most modems come configured for a certain setting. If this doesn't work and causes conflicts, you'll need to change it. Most modems have jumpers, and you simply move the bridge(s) to cover whatever pins are required for your setting. Again, consult the manual for the jumper configurations. Other modems have DIP switches. Newer modems are plug-n-play and will configure themselves, but a good plug-n-play modem will have the jumpers just in case.

During installation, you will be asked to select what type of modem you have from a list. Try to pick the exact modem. If it isn't there, try picking one as close as possible, such as the internal version of the same modem for an external modem. If you can't find any of these, usually picking a standard Hayes compatible setting will work. This will usually get you online so that you can look for better configurations at the manufacturer's Web site or BBS.