Here's how to Snap Off Blade Cutters, along with important safety precautions:

Safety First!

Always wear safety glasses or goggles. Small fragments can fly off.
Wear cut-resistant gloves if possible.

Never snap the blade directly with your fingers, even with gloves.
Work in a well-lit area.

Have a designated disposal container ready. This could be a specialized blade disposal case (recommended), a hard plastic container (like an empty pill bottle), or a metal can. Do not just throw it in the trash, as it can cut through bags and injure sanitation workers.
Methods for Snapping a Blade:

Most utility knives with snap-off blades have pre-scored lines that indicate where the blade will break.

Using the Built-in Blade Snapper (Most Common and Safest):

Many utility knives have a small, removable cap or tool at the back of the handle. This is often designed to be a blade snapper.
Extend the blade until one dull segment is exposed beyond the end of the knife, up to the next score line.
Remove the blade snapper from the back of the handle.
Insert the dull tip of the blade into the slot on the blade snapper, aligning it with the score line.
With a firm, quick motion, push the snapper down or away from you. The blade segment should snap cleanly off.
Immediately dispose of the snapped-off piece in your sharps container.
Reattach the blade snapper to the knife.

Using Pliers:

If your knife doesn't have a built-in snapper, or if you prefer this method, pliers are a good alternative.
Extend the blade until one dull segment is exposed beyond the end of the knife, up to the next score line.
Hold the exposed dull tip firmly with a pair of pliers, making sure the pliers are parallel to the score line.
Hold the utility knife tightly with your other hand.
Snap the blade downwards or away from you at the score line.
Immediately dispose of the snapped-off piece in your sharps container.
Using a Blade Disposal Case (Specialized Tool):

These are containers specifically designed to safely snap and store used blade segments.
You typically slide the dull blade tip into a slot on the case, aligning it with the score line.
Then, you apply pressure (often by pushing a lever or the knife itself) to snap the blade, and the segment falls into the case. This is a very safe and convenient method for disposal.
Important Considerations:

Don't try to snap off too much blade at once. Stick to one segment at a time.
Ensure the blade is firmly locked in the extended position before snapping.
Keep your fingers clear of the blade's path and the snapping point.
Dispose of broken blades responsibly. They remain extremely sharp and pose a significant hazard.