tuanbusku

I'm used to old Serif ways of thinking, so can anybody straighten me out on this Affinity Designer problem? I imported a wide jpg as a tracing template. SUbsequently, I changed the layout to a rough square, but I'm stuck with the wider format. I want to trim this to fit the actual graphic which only takes up half the "canvass". I can't for the life of me see how Designer allows you to do this, although it seems such a basic task. Help!