Don't know where to start? Start here.
This is a work in progress. Please post topics you would like to see covered in the Comments.
3D Printing Basics
Materials
Adhesion
Bed adhesion is key to 3D printing. Very simply, if your print doesn't stick then your print will not be successful. If your print sticks too well you can destroy your print or your printer trying to get it loose.
The optimal printing surface (for PLA, PETG, and ABS) is glass. Period.
Printers
Leveling Sensors
Having (re)built a Prusa i3 from a kit, I realized some important lessons that anyone who started with a retail printer will have missed.
My i3 came to me with an 18mm inductive bed sensor. It did a capable job of detecting the circuit board heated bed. But I noticed that sometimes it could go out of adjustment by a pretty significant amount for no apparent reason.
So I added bed adjustment thumbwheels. They seemed like a good idea, but as they say, with a bed leveling sensor these are as useless as tits on a bull. Bed leveling adjusters give you the ability to make your bed perfectly parallel with the X-axis rods, and this is useful. However, you adjust this once and should never have to touch it again. Shims are a better approach. Using these adjusters to adjust the nozzle clearance is completely pointless, as the inductive sensor does that. Move the bed up and the sensor moves the nozzle. Worse than being useless, my setup cost me over 20mm of Z-axis height. I removed them.
Now back to running just the inductive sensor, and having upgraded to a beefier aluminum plate for the y-carriage, I experimented with a 12mm inductive sensor so I could reduce the footprint of my print head. Even the best 12mm inductive sensor has a 4mm sensor distance for steel. Aluminum reduces that distance by more than 50%. My glass bed surface is 3mm. The 12mm would hit the glass before it detected the plate.
So, back to the 18mm inductive sensor, and all is working well...until it isn't. After a 12 hour print I started a new print. The sensor was suddenly off by at last 1/4 of a mm for no clear reason. After over an hour of fiddling with it I discovered the reason (and recall reading about this): the temperature of the probe/bed affects the detection distance. When I adjusted the nozzle clearance on a cold bed the clearance on a hot bed was too high (cold requires closer detection distance). When I adjusted it hot the nozzle would hit the glass on cold detection. Worse, if I didn't preheat the bed for at least 10 minutes the detection distance was unreliable, and even worse yet, could be different across the three detection points my printer uses.
My next "upgrade" is completely counter-intuitive: I am removing my inductive bed sensor and throwing out the entire idea of automatic bed leveling in favor of the three-point manual leveling used by printers like the FlashForge Creator 3D and Monoprice Select. I intend to install a z-min stop switch on an adjustable mechanism that will allow adjustment of the switch to set the bed height. The bed itself will be leveled manually using a digital caliper to ensure that it is parallel to the X-axis, and shims to adjust it to be level on the y-axis. X-axis adjustment should be done before each print (very easy - simply turn a z-axis lead one way or the other until the sides are equal). Y-axis adjustment should only be needed once.