There are many approaches to levelling the printing bed on a cartesian printer.
Here is one approach!
To do this you will need to be competent in adjusting your printer hardware.
This post is relevant to those printing in PLA. If you are using other filaments you will need to check if these arrangements suit your choice of filament.
A really flat bed is the best thing to start with. A sheet of boro-silicate glass is ideal for this and can be purchased from internet shops. You may also research alternatives.
We assume that the glass is reasonably flat, to a better tolerance than a metal heated bed, and show less distortion when heated. The glass is simply placed on top of the metal heated bed.
A useful way of attaching the glass to the bed is with Bulldog Clips (UK name) or binder clips. Usually four of these are used.
When you add the glass bed, you need to adjust your Z-axis to take count that the Z zero homing position is now higher. This will be roughly the thickness of the glass sheet.
A means of adjusting the bed, usually there are adjustment screws at each corner that compress and relax a spring, so that the tilt of the bed can be adjusted. Sometimes this is by wingnuts underneath the bed. My option is to use a pronged nut into the plywood carriage and a thumb-nut into the top. For me this makes adjustment of rear left and right easier.
A digital level is also a must. These can be inexpensive from the internet shops. They do have a low refresh rate, so you need to be patient when using these to let the reading settle for a while. It usually takes around 30 seconds, which seems like ages.
The rest is a downloadable PDF.