For the story, it depends on if it's an image set or if there's some comic panels already. If there's existing images with dialog, then the story's already kinda there and I just need to fill in the blank spaces, get from A to B and add more as necessary.
With an image set like this it's much tricker, and mostly comes down to perversion and indulgence dreaming up some kinda plot line which I then have to make sense of. As an example, the plot for this came from how May's facial expressions imply reluctance turning into overwhelming pleasure in the first half, and then just expanding on the idea from there. From there it's just a matter of writing the dialog, which involves a lot of me going "is this line better suited on another page and/or can I actually fit all this dialog without covering up the fun bits".
Either way, the real tricky part is motivation, which is why my output is so hit or miss. If there isn't any clear inspiration, I can't get anything done. That and it takes maybe 10-12 hours to do something like this between setting everything up in Libreoffice, writing/editing the dialog and such.