I finally bought a 3D printer, one of the cheap, well-trodden Ender-style machines, and the first thing it printed was perfect. The little test boat, the Benchy, came off the bed clean and crisp and I sat there feeling like I had cracked it. This is, I now understand, the printer lulling you in. The first print uses the profile someone else dialled in. Everything after that is yours.
The second print was a clip for a cable, a trivial flat thing, and it failed three times. The first attempt would not stick to the bed and the nozzle dragged a tangle of filament around like a cat with wool. The second stuck on one side and lifted on the other, warping into a banana. The third produced a sad nest of stringy plastic because I had not noticed the first layer never adhered and the printer cheerfully kept extruding into mid-air. Spaghetti, as the forums lovingly call it.
Almost every one of those failures was the same failure wearing different clothes: the first layer. Get the first layer to stick flat and even, and most prints succeed. Get it wrong and nothing downstream can save you. The fix was tedious and entirely mechanical. Level the bed properly, not with the screws-by-feel approach I had used, but with a sheet of paper under the nozzle at each corner, adjusting until the paper drags with just a little resistance. Then set the nozzle height so the first layer is squished slightly into the bed rather than laid gently on top of it. A first layer that looks a touch too flat is correct. One that looks like neat round noodles is too high and will not bond.
The other half was adhesion and temperature. The warping banana was the plastic cooling and contracting at the edges and peeling off the bed, so I dropped the print speed for the first layer, nudged the bed temperature up a few degrees, and wiped the glass with isopropyl to get the grease off. PLA is forgiving once it actually sticks. It just has no patience for a dirty or cold or badly levelled bed.
By the end of the week the cable clip printed first time, every time, and I had stopped feeling clever and started feeling like someone who understands roughly one thing about a machine that can do hundreds. That is the right amount of humble for week one. The boat was the printer's victory. The clip was mine.