What the heck is Print-on-Print? It is a technique for embedding crisp, high-contrast graphics in the first layer or two of your print. In this case, the entire print is only two layers thick, so the graphics appear on both sides of the bookmark.

It works by having your graphics as a separate object which is only one or two layers thick. The graphics object gets printed first. Then on top of that you print the rest of your object, which has a zero-clearance cutout in the first layer or two for where the graphics object goes. The two objects print touching one another, fusing themselves into a single object. Note that if the graphics object is taller than 1 or 2 layers, the nozzle will plow into it significantly when you print the main object. Here, I've provided Celestial-Bodies.STL as the graphics object and Celestial-Background.STL as the main body of the bookmark.

Unfortunately, in Prusa Slicer (as far as I can tell) you have to play a few tricks to make this work. Firstly, to keep the two objects aligned, I had to create a bounding box around both so that when Prusa Slicer centers the objects, they stay aligned. (Prusa Slicer doesn't seem to give you access to the original X-Y position from the STL file.)

Secondly, you need to make some minor edits (shown below) to the original G-Code files to execute the filament color change, to keep the motors from turning off, and also to prevent the printer from re-homing before printing the second object. (Homing is not exact and if you re-home between prints, there will be some minor misalignment.)

Printing
Here are the exact steps you need to take:

1. Load the Celestial_Bodies.STL and center & align the object.
2. Slice the object using 0.2mm layer thickness and save the G-Code file. (Also make sure there is no skirt, and that the elephant's foot compensation is set to 0.)
3. Edit the following lines in the G-Code file (found at the end of the G-code):
; comment out this line: M104 S0 ; turn off temperature
; comment out this line: M140 S0 ; turn off heatbed
; comment out this line: M84 ; disable motors;
; Add this line directly following the M84 line:
M600 ;Execute filament change
4. Load the Celestial_Background.STL and center & align the object.
5. Slice the object using 0.2mm layer thickness and save the G-Code file. (Also make sure there is no skirt, and that the elephant's foot compensation is set to 0.)
6. Edit the following lines in the G-Code file (found at the start of the G-code):
; comment out this line: G28 ; home all without mesh bed level
; comment out this line: G29 ; mesh bed leveling
7. Print the Celestial-Bodies file first. You may get a warning from the printer about the file not being for your printer, but ignore this and continue. The graphics will print, and after printing you will be prompted for a filament change. After the filament change, the printer will park itself, but the motors will stay on and the bed and nozzle will also stay hot.
8. Print the Celestial-Background file. You may get the same warning as above from the printer, which you should ignore. This file should print without the printer re-homing or re-executing the bed leveling.