Step 1:
Import your line drawing. I recommend a size at least two times the size of how you will want the end result to be, depending on the complexity and detail you wish to put in. Ensure that all of your lines are connected, and there are no white spaces in your lines, otherwise you will have to fill them in with the paintbrush in the following steps, and that is a hassle.
Go to Layer -> New -> Layer from Background. Add another blank layer to that.
On the layer with the line art, select Channels as per the diagram below, and click the dotted circle icon to select all of the lines. Hit backspace/delete to delete the white areas.