Here's that Poison Ivy step-by-step (sort of), that I mentioned earlier.
I used photo ref in this case, but I prefer not to, unless I get stuck on something like an anatomy problem, or foreshortening. I also use default photoshop brushes, at varying opacities, set to either normal, darken, multiply, overlay, or color. I like to switch between the pencil and the brush tool.
From time to time, I flipped the image, horizontally, to see if anything was too wonky. If so, I'd try to fix it, then flip the image back. After the greyscale was finished, I converted to RGB, then tinted/painted the color.
All told, this illustration took about 3.5 to 4 hours.
edit - I didn't realize I was limited to 3 attachments at a time. So, the final color illustration is on the next page.

- Poison Ivy spread 01.jpg (47.99 KiB) Viewed 2900 times

- Poison Ivy spread 02.jpg (56.93 KiB) Viewed 2900 times

- Poison Ivy spread 03.jpg (103.43 KiB) Viewed 2902 times