We use this punch tool on the insulation displacement connectors: https://www.digikey.com/product-detail/en/avx-corporation/069176701602000/478-4623-ND/1650845
The connectors are designed to use with this tool. As @johng and @Tangaroa mention, you might be unsuccessful or even cause damage without this tool. Without the tool, it might help to place some wadding like folded up paper towel over the wire before pushing it in order to get some conformation and even pressure application.