Step one: Find the Stud
I have found the most reliable way to find studs, is with a medium to strong magnet. The magnet sticks to the screws that line the perimeter of drywall sheets. These screws secure the drywall sheets to the framing studs of the wall. Find the screws in the drywall, you found the studs behind the drywall.
If you are having trouble finding a stud it helps to know where they are supposed* to be. Studs are almost always 16" or 24" inches apart from each other, 1.5 inches wide, and every light switch or wall outlet should* have a stud directly to the side of it on the left or right.
Once you've found a drywall screw you should be able to find a vertical line of them, this should give you a pretty decent idea where your stud is, mark the left and right edge of the stud with a pencil.
If you are not able to find a stud or need your shelf somewhere where none exists, It is highly recommended you use some quality drywall hangers. If you don't have any on hand, I have provided some that you can print.
Place your shelf on the wall where you think you want it, use a bubble level or your phone's app, and trace a line where level is. Your first screw should now be placed somewhere on the line and in the middle of your stud.
When installing your first screw, do not worry about keeping your shelf level, as this is can be adjusted when installing the second. Instead, focus on the angle of the screw matches the shelf hole angle and be sure not to over-tighten the shelf. If you over-tighten the screw it can crush the 3d printed plastic shelf. It is better to be a quarter turn loose, than a quarter turn too tight in this case.
If you want to be extra secure your second screw can use a drywall hanger, but if you found a good stud on the first it is not necessary.
Now that you got the first screw in, your shelf should still be just loose enough to adjust to make it level.
Install your second drywall screw, and like the first be careful not to over-tighten, but in this case we are trying to avoid stripping out the drywall (or drywall hanger)
Once you are all done,
Best practice for adding books:
Use your largest hardcover book as the base
Stack books heaviest to lightest, and largest to smallest.
Note, very large books (textbooks) will have a center of gravity beyond the edge of the invisible shelf, and will not stay on the shelf on their own. Adding smaller books on top will keep it in place but these large and heavy books will cause the shelf to sag more than a small heavy book will. be careful and use your best judgment when is too much for the shelf to handle.
Note, that the FDM-printed plastic is semifluid and very susceptible to heat. heavy loads in hot environments will cause the shelf to slowly fail over time.
My recommendation is to use smaller stacks of books and print a bunch of shelves for the best results.
Congrats, thanks for following along, and enjoy!