Navigate to the Space that you want to copy. If you want to copy the whole space, navigate to the Home Page of the Space. If you want to create a new space based on a Page Tree that exists on the space, navigate to the root page of the Tree, in this case the root page will be the Home Page of the newly created space.
Instantiate the copy process:
Enter a name and key for the Space that will be created, choose if you want to define a prefix to be added to all pages copied, leave it empty to add no prefix. At the end choose if you want to copy without replacing, if this is checked, the replace placeholders dialog will be skipped and the placeholders will remain on the newly created Space. If you leave copy without replacing unchecked and skip the replace placeholders dialog manually, the placeholders will be replaced by blanks:
Click on Create, wait for the Copy process to finish and redirect you to the newly created Space.