How to make 60X90 posters and A4 flyers with PS? Seek an answer

When creating a new document, you should pay attention to the color category (of course, the printed things should be -CMYK) and the pixels should be 300px (resolution 300). Enough!

This is a reference X bookshelf, the size is 6x 16CM (originally it was 60x 160, so I scaled it to 6x 16 because it was too big to upload). It can be designed as 60x90 when it is built!

I don't expect you to adopt it, I just hope I can help you!