ISA modem hardware-installation is just the same as for PCI-one. ISA bus is slower than PCI, thus PCI slots have replaced ISA slots these days. It could well be that in the future the same happens for PCI slots, but that's another story.
So, just locate an ISA slot in your motherboard, take the card and plug it in (just like you do with PCI cards). When it comes to the software installation (which you probably meant rather than hardware), it's pretty similar to pci modem installation: if your kernel detects the device, it either automatically creates a device file for it (newer systems) or on older systems you'll create it yourself using the userspace tools available. ISA modems are so old nowadays that I believe/hope they're mostly "real modems", and don't need special per-model drivers like these newer "software modems" or "winmodems" do. Therefore you shouldn't have much problems. On somewhat new operating systems you can probably just shut your machine down, plug the card into the ISA bus, boot the machine and have your system detect it for you - then just run your favourite ppp connection configuration program (kppp, gtkdial, ...) and setup the connection. A common link to the modem device file is /dev/modem but it may differ.