Wear safety glasses and a respiratory mask to protect yourself from concrete dust and debris.
Use a sledgehammer to loosen the concrete around the piece of rebar. Hit the area around the rebar as many times as necessary to get the concrete loose enough to remove the rebar.
Pull large chunks of concrete out of the area around the rebar.
Use a stake puller to pull the rebar out of the concrete. Place the stake puller so that the lever faces you and the notch is at the base where the rebar is level with the concrete.
Push on the stake puller lever to lift the rebar out of the concrete. If all of the concrete is crushed, you may be able to wiggle the piece of rebar enough to loosen and pull it out without a stake puller.