There are a couple ways to do this but if you do this a lot, the easiest would be:

(1) Click on the Prints tab top right of the main UI.
(2) Right click in the middle of the print size list and choose "Add a new size".
(3) Enter 40x60 as the size (optional: you can give it a short name if you like).
(4) Under the list of print sizes, enter "5" as the "Inner" border and click on the color swatch to the left of the border size to choose the border color.
(5) Still in the "Borders" area, make sure "B" is checked (not B+).
(6) Click on the "Printers and Settings" top right and drop down "Cut Marks" and choose "Guide Lines".
Note: the auto cropping button to the left of the "Borders" should be on/highlighted so that auto cropping is turned ON.

After doing the above, you can select any thumbnail(s) you like and then click on the newly added 40x60 size in the size list and they will be sized as 40x60 prints with a 5cm border on all sides, giving you 30x50 prints inside the borders.  The guide lines will print at the edge of the 40x60 borders.  You can also save as a printer setup and name it something like "30x50 in 40x60 frame" so you can recall that at any time and skip the above steps.  Or, now that you've added the 40x60 size to the size list, just enter "5" for the inner border and choose your 40x60 size.  And again, the auto cropping button (scissors icon bottom left of the Prints tab) should be highlighted/on so QU has permission to crop images to a 3:5 aspect ratio (your 30x50) if they are not originally that shape.

Regards,
Mike