I would clean the bed with alcohol to make sure anything is off of it. Finger prints can cause trouble!
Level, level, level and level again. When doing the level with the paper, use the control panel on the printer which I assume you are. I head my bed to the temp I will be using it at first.
Set the nozzle you will be using directly over the adjuster. Check and turn the adjuster. Make sure to let go of everything when checking with the paper so your hands don’t interfere. Keep checking each of the three points until you can check all three and not make any adjustment. You will get used to this.
Glass can be very good for both ABS and PLA. Hair spray (aqua net super hold purple) or purple glue stick. Google on how to apply but generally cold bed. You will also need a shim for the Z axis limit switch which is behind the plastic plate on the bed with the logo. Check thiniverse.com and search for flashforge.
You can add the glass on top of the blue sticker. You will need to clamp it down in some fashion. Many ways to do it. And yes, leveling done with glass in place and again I would let it heat to operating temp you will be using giving the glass time to reach temp and stabalize.
Temps: PLA Generally - 180-220, most people run 190-210. Bed can start at 65 but drop a bit after the first layer. You can do this also with the extruder temp but don’t worry to much for now.
ABS 230 is a good target. Bed a max of 110 but 100-105 should be good.
You may find you need a little more temp with glass but it just takes time to see.
I recommend Borosilicate glass. Google for it for a 3D printer. eBay, Amazon, McMaster-Carr.
Heating it generally not but it is glass.
Don’t get fancy with prints to start. Get a calibration cube and learn how to do the calibrations. Google for many examples.
Also, run flashprint and make sure your firmware is up to date. Sailfish 7.8
