Skidding/skipping a fixie with ratio 48/16
the easiest way to start learning how to lock your legs correctly is to alleviate weight to rear wheel. The less weight on the wheel the easier it will be to skid. Lean forward (BE CAREFUL) and then try to lock your legs, think about where your legs are the strongest and try there.
Many people also do "hop skids" where you hop your rear wheel (lift it off the ground) and lock it while it is in the air, then return it to the ground while it is locked.48/16 isn't a bad ratio (pretty normal). you don't wan to dictate your ratio based on your skidding, you want it to be based off your riding cadence.