what steps to take after removing a cable from a breaker box?
You can plug the knockout hole with a knockout seal (or knockout plug).


(source: homedepot.com)  

You'll likely need either a 1/2" or 3/4", depending in the knockout size.

WARNING: When working in the panel, take your time, be careful, and manage loose wires and hands to keep them away from energized parts. Be especially cautious of the main lugs, and the ungrounded (hot) bus bars. Use insulated tools, and wear appropriate safety equipment.