The screenshots below are from Mac OS 7.5.
The computer must have a Modem already installed and set up.

Mac TCP/IP is available from the Control Panels. Select the PPP icon.
Make sure the IP Address is 0.0.0.0

Then click More. Then this window will open:

Click on the Manually button in the Obtain Address section.
Now select A from the Class pull down menu.

Domain: Type a full stop in the first two boxes.

IP Address: Type 212.85.249.130 in the first box. In the second type 212.1.130.32. (Make sure that next to the first box you selected Default, as in the picture below).

Gateway Address: Type 0.0.0.0 in the box.

Now select Server from the Obtain Address area. Press the OK button, and close TCP.

Now choose Config PPP from the Control Panels folder.

[Note : this window might look slightly different, but everything should work the same]

Next, click the button New

Type Connect as the PPP server name

Click OK

Click the Config... button:

Then modify all entries to make your window match the one pictured below, except the port speed and phone number. Enter the dialup number ( Modem number : 0845 6600971, ISDN 64K number : 0845 6651583, ISDN 128K number : 0845 6651584 ) and select the correct port speed for your device.

Click Connect Script...

You should set your window to look like the picture above, but do not forget to replace "-ENTER USER NAME HERE-" and "-ENTER USER PASSWORD HERE-" with the provided user name and password. When finished click the OK button. Now you can connect to the internet.