I print on a mirror tile, 12" x 12".
I use Elmers School Glue Stick (disappearing purple - jumbo size).
I put a fairly dense coating on the glass bed. I try not to leave any unglue stick covered glass where the print will be.
Then I take a small spray mister with water, and spray just 1 or 2 times to get the glue stick wet and purple all over.
This way the glue dissolves and becomes evely spreadable.
I use a 2" or 3" paint brush I got at Walmart. Its just a standard paint brush.
I wet the paint brush with the spray mister and spread the glue over the entire glass bed.
I make sure to get it all dissolved and evenly spread out.
Let it dry for about a minute or so, PLA sticks so well to a freshly glue stick brushed glass.
Just glue stick works great too but its hard to get it evenly applied after a few prints.
When you need more stickum, just add some glue stick to the end of the wetted paint brush and repeat as needed.
Hope this helps! :)