1) Open a new document on Photoshop size doesn’t matter.
2) Press B on your keyboard to select the brush tool.
3) Now, press F5 on your keyboard to bring up the brush menu.
4) Change your brush spacing and you can change your hardness to make it look like this:
5) Then, you are done with that window, press F5 to close it You can now go ahead and draw your line on your picture (or whatever document is open).
6) Here’s my finished product.