Empty it once
Pull out the bottles, towels, and random extras. Check for dampness, odors, stains, or bubbling laminate. If you find active moisture, identify and fix its source rather than simply covering it.
Measure the usable footprint
Pipes, hinges, and cabinet supports can shrink the available area. Measure the inside dimensions at the bottom, not the outside of the cabinet.
Group supplies by purpose
Keep frequently used items in front. Make sure harsh cleaners remain in their original labeled containers and out of reach of children or pets.
Think of a liner as insurance, not a repair
A raised-edge mat can help catch small drips and makes spills more visible. It will not stop a leaking pipe or prevent all water damage. Inspect the area periodically.