"Coast Lines"?

[Beoner]

Hello,

So I am working on a quick map here while I am busy with study, and I wanna do some perpendicular lines, or something like that , on my coast (You'll probably understand it better if you take a look at the picture). I remember I saw how to do it on a tutorial here, but it was a long time ago and I can't remember the tutorial nor find the tutorial.

So the question is, Does anyone here know the link to the tutorial, or at least who made it? If anyone knows how to do it and wnat to explain I would be grateful the same way