Today I installed a glass bed on my Monoprice Select Mini 3d printer. I did it a bit differently from most of the conversions I've seen online.
I purchased 3/32" inch (2.38mm) glass from Home Depot, and cut it to size with a carbide glass cutter. Next I used a knife sharpening stone and some vegetable oil to grind down the cut and factory edges so they'd be safe to handle. I rounded off the corners a bit too. With that done, I peeled up the bed tape from my aluminum bed, scraped off the high points of the bed's dings and scratches with a razor, and set the bed in place. I held the glass in place with 4 Wal-Mart 1/2 inch Mini Binder clips.
This is, so far, fairly standard stuff. If you were following most online tutorials the next step would be to install a clip around the X axis gantry to raise the Z axis home position by the height of the glass. I did something different. I raised the hot end instead. The hot end is held in place by a 1.5mm grub screw. I removed the fan, loosened the grub screw, took out the print head, removed the PTFE spacer block, reinstalled the print head, raised it well above its old height, and re-tightened the screw. Then I carefully homed the printer, loosened the screw, lowered the printhead into the correct position, and retightened it. Viola, Glass print bed and no spacer required.
There is a downside to this approach. That PTFE tube spacer helps filament feed smoothly back into the print head if you retract it too far. I'll be replacing this very soon as part of my NinjaFlex fully-constrained-filament-path upgrade.
It's worth noting that I didn't intentionally choose to do my glass bed conversion this way. I printed a glass bed spacer from Thingiverse (https://www.thingiverse.com/thing:1621650 ) and disassembled the printer to install it. Unfortunately I found that it didn't snap on to my x gantry properly, and was "sprung" into position such that it would never be accurate on my printer. That's probably a printer calibration problem on my side, and not a problem with the model. This way appears to be working, so I'm happy with it.
Tiny gnomes stole one of my fan clips while I had this apart. I foiled their evil plot though; I fabricated a serviceable replacement from a paper-clip.