Thanks to the folks here for terminology and leads. @Yiannis' answer is indeed the professional tool. I want to leave here that I've also stumbled across a version of the IDC hand tool that is occasionally sold with a DIP block die, like so:
It's much cheaper than a Panavise setup though the reviews are not stellar.
Those expensive press tools that Yiannis shows are the "official" way to attach ribbon cables to those IDC connectors. They're not strictly necessary, though. All you really need is the ability to press straight down on the connector at a 90 degree angle.
Channellock pliers work better on IDC connectors than normal pliers since their fulcrum is ...
As others correctly point out, you can use pliers or a small vice. I have used a heavy book and just push hard on it, but there is always the risk of breaking the connector or getting hurt (have done both).
However, the "proper" tool for such connectors is a table press, and like most professional crimping tools they get expensive fast. They look ...