Hi Brampage,
Welcome to Affinity Forums :)
Do you mean laying out/designing websites graphically (generically speaking) with any app or using Affinity Designer specifically for this task?
I'm not aware of any tutorials dealing with designing websites in Affinity Designer but you can easily adapt/replicate tutorials done for other apps using it.
Most of them, however, focus on building a design (technically) with a specific app and not on the design process itself. I will see if i can found any tutorials about web design using Affinity Designer. Hopefully someone will add a few more resources too.
