13- Use eraser tool set to about 30 soft pixels and opacity set to about 50% and erase unwanted ink clumps knots and unwanted anomalies, turn off layer visibility

14-Repeat steps 8-13 at least 2 more times and turn on visibility for all layers

15-Link all layers together except for your base image layer and Merge Linked, this is now your cracks layer

At this point you have options as to what to do with your cracks layer. You can stop here and set layer mode to Multiply and reduce layer opacity to about 30% merge with your base image and you're finished.