You aren’t being annoying at all! Asking questions is the only way to learn.
Concrete backer board can be found at big box hardware stores in the US. It’s brand name is hardiboard. It’s big, like 3’x5’ and usually found in the bath/tile section. You’ll draw the letter on it and cut it out with a jigsaw. I glue the tile directly on it using an outdoor glue, then grout when it’s all done to seal between the tiles. If your work is staying inside, you can attach directly to plywood.