Now scan your marks into Photoshop, then go to Image > Adjustments > Desaturate to make everything completely black and white. Finally, go to Image > Adjustments > Auto-level to make the whites brighter, and the blacks darker.
With the Lasso tool, roughly draw around one of the marks you made, then go to Select > Color Range. Set the fuzziness to 200, then use the colour picker to click on the darkest part of the stroke you’ve selected.
To turn the selection into a Photoshop brush, go to Edit > Define Brush Preset. This brings up a window where you can assign a name to your brush if you want. Your custom brush is now in the Brushes palette. Repeat this process for all of your scans.