How do I apply BNC connectors without breaking them?
There's a large variety of compression tools and connectors, and unfortunately they aren't all compatible with each other. Most likely you'll need to use a different tool or connector style to make a proper compression fitting. But a few tips from the last time I was installing coax connectors:


Make sure the tool is properly adjusted, especially with the depth.
Fully seat the connector inside the tool so you get 3 sides secured where the cable exits the tool.
Select the correct die if you have a choice and properly align the end of the connector with the tool.
If you aren't sure of the depth or start to warp or bend the connectors, only partially compress the fitting, spin the cable so other sides are being stressed near the cable, and adjust the depth until you have reached full compression but no further.