So, I found these instruction for Adobe photoshop to create a pencil sketch, but I don't know why they don't work in Affinity Photo. Simply, you create layers to Desaturate, Invert in Color Dodge blending mode, apply Gaussian Blur, and the slider can give you anything from a faint sketch to a nice value study. Plus, another adjustment layer - levels helps strenghthen it. But I can't get past the Gaussian Blur step with Affinity Photo. Here is the link to the instructions if you'd like to try it: https://www.adobe.com/creativecloud/photography/discover/photo-to-pencil-sketch.html . I've been trying to avoid using P/S, but I've had to keep it on my desktop since there still seem to be things Affinity can't do.