This tutorial covers how to use any of TheVectorLab Graphic & Logo Templates in Affinity Designer for Desktop. Go here to see the tutorial for Affinity for iPad.
Getting started with Graphic & Logo Bundle Vol.1 for Affinity Designer for desktop Mac & PC's. (link coming soon!) These designs and fonts can be used for designing t-shirt graphics.
Using the templates in Affinity Photo is essentially the same process.
You can find the Affinity Designer for iPad Tutorial Here.