Where should I put a scratching post?

Right next to wherever she’s already scratching, not in the corner you’d prefer it to live. A post she ignores is just expensive furniture. Cornell’s guidance is to place the post next to the area she’s currently using, then gradually move it (a foot at a time, every few days) toward a less central location once she’s reliably using it. If she scratches in multiple spots, you need multiple posts. One post per area she targets, plus one in the main common-area, is the rule that works.