From the menu select File - > Print or press ctrl + p to bring up the print dialog:
The print dialog is divided into three sections:
1. Lets you print the current page or a page range (only visible if the document has multiple pages).
2. Lets you specify the size of the diagram with respect to the given page size. Either a zoom value can be set in the Adjust to field, or a number of pages may be specified to fit the diagram into (where a value of 1 for both fields fits the diagram to a single page).
3. [Advanced] Overrides the paper size and sets the size of the page relative to the given diagram size. draw.io doesn't have access to the operating system print settings. Sometimes what draw.io thinks is A4, for example, isn't what the OS thinks, or when you select landscape you have to manually match that setting in the actual print dialog of the OS. Page scale is only used when you need to match the page size in draw.io to that of the system.
Next, press print to go to the browser print dialog, to select the print device, etc.