Clean Up Your Traditional Drawings background in autodesk sketch book

hello guys, this time I am gonna show you how to clean  Up Your Traditional Drawings background in Autodesk sketch book.

Select File then Open – Grab an existing sketch you want to clean up 

Tap.Select and Select the polyline tool

Click to add points to the polyline, then click your first point to complete the selection.
like the pic 

Tap Invert to switch the selection. The current unselected content will become the selection then press the Delete key on your keyboard.

Tap Deselect to deselects everything 

Tap  Magic Wand tool can select an area of similar shade or color.

Then press the Delete key on your keyboard or use the eraser to clean up the gray area.

Et voila nice and clean background image.

Final look

you can watch the video to understand more 

Source: https://www.sketchbook.com/help/knowledge-base/selection-tools/.