Nothing in this hobby has eaten more of my evenings than the first layer. The print itself is usually fine. It is the bottom millimetre, that fragile handshake between nozzle and bed, that decides whether you get a part or a tangled mess you scrape off whilst muttering. For months I treated levelling as a ritual rather than a problem, and rituals do not survive a glass bed that is slightly bowed in the middle.
the paper years
The classic method: heat the bed, slide a sheet of paper under the nozzle at each corner, turn the knob until you feel a slight drag. It works, in the sense that it is better than nothing, and it taught me what "right" feels like. But it assumes the bed is a flat plane, and mine simply is not. Corners perfect, centre low. Every large print told me so.
probing the truth
The thing that actually ended the war was an inductive probe, in my case a BLTouch, and turning on mesh bed levelling in the firmware. Instead of trusting four corners, the printer touches a grid of points across the bed and builds a height map, then adjusts Z on the fly as it prints to follow the surface.
In Marlin the relevant bit looks roughly like this, with the grid set before each print:
M420 S1 ; enable bed levelling
G29 ; probe the mesh
M500 ; save the mesh to EEPROM
I run a G29 as part of my slicer's start G-code so it remeshes when the bed is hot, because a cold mesh and a hot bed are not the same shape. That single change did more for my success rate than any amount of paper ever did.
the bit nobody tells you
The probe gets you a good map, but Z offset is still on you. Too high and the lines do not bond; too low and you are ploughing the PEI sheet. I dial it in live with babystepping during a first layer, watch the lines squish to just-touching, and save the offset. A square of single-layer test print is the honest examiner here. If it peels off clean as one sheet, you are done.
I will not pretend the war is over. A new build surface, a slightly loose grub screw, a draught from the garage door, and the front comes back. But it is a skirmish now, not a siege, and that is the most you can ask of a machine that flings molten plastic about for a living.