- Draw a rectangle covering everything you want to copy
- Press Ctrl+A to select all objects
- Object > Clip > Set
- Only the parts that were covered will be left
- Select all objects you want to center
- Object > Align and Distribute (or Ctrl+Shift+A)
- Choose 'Relative to: Page', tick 'Treat selection as group'
- Press the third buttons in both the first two rows of icons (center horizontally and vertically)
- Create a new layer with Ctrl+Shif+N
- Move objects between layers by selecting them and pressing Shift+PgUp/PgDown.
- Change layers using Ctrl+PgUp/PgDown