Here's an example: https://en.wikipedia.org/wiki/Origin_of_Hangul#/media/File:K...

Here's another one that shows standalone jamo: https://en.wikipedia.org/wiki/Origin_of_Hangul#/media/File:H...

So, basically, all the consonant jamo were simple geometric shapes, with more straight lines and fewer curves. For vowels, what is today treated as a line with a stroke was actually a combination of a line and a dot (the dot itself being a glyph for a vowel that has since disappeared from the language).