1- Open Affinity Designer.
2- Open a new document and draw a shape/design/image.
3- Add a new layer.
4- Select the vector brush tool and draw where you would like on your shape/image/design. you can go over the edges as we will use the vector crop tool to remove.
5- With the move tool select everything.
6- Layer > Group.
7- Select the vector crop tool and select your shape/image/design. Once selected you can now move the lines to remove the lines that are over your design.
8- Once you have moved the vector crop area and moved it in your excess lines will now be gone.