For iPad apps, look up Astropad or Duet Display. They work best with the Apple Pencil but you can still use a passive (cheap) stylus or your finger to draw on the iPad inside of the Affinity apps.
Hello and welcome to the forums. Sorry I don't know of a link to a video tutorial for how to prepare a photo for output but I do have a few questions. Are you a printer? I have worked in design and printing for a long time and normally the printer or whoever is going to make the film or the plates (or in your case the screens) deals with things like frequency and the angles. So if you aren't the printer, don't worry about it. If you are the printer, it begs the question, how are you a printer with no experience with Photoshop? My second question is, are you using a continuous tone image (a photo) or something like a logo with text? A lot of, if not most screen printing artwork would be better done using a vector program like Affinity Designer. If you have no experience with Photoshop you might not be completely aware of the differences between raster images (Photo/Photoshop) and vector programs (Designer/Illustrator)? Most screen printing shops use more vector art than raster art. Of course they do use raster art but in general they try to use vector art if they can.
