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To prep a small crack rinse it out with water and allow it to dry thoroughly. Also use a steel scrub brush to knock any loose particles away from the crack. If there is any loose pebbles or particles in the crack or hole use a shop vacuum to remove them.
Use mortar repair in a caulking tube to fill in the small cracks, forcing it into the crack to seal it and prevent water from leaking into the concrete. The tip of the caulking tube will smooth the mortar repair a bit; you can do the rest of the smoothing with your finger.
To repair larger cracks or holes, make a batch of concrete patch, which you can buy in small bags at a home or hardware store. Mix it according to the manufacturer's instructions.