Hanging a mirror using mirror mastic

i have recently bought my second mirror. My first one fell off the wall in the middle of the night. I had used liquid nail to hold it up and used two bracers to hold the mirror into position for over 48 hours. 6 hours later it fell with nobody in the area since we were all sleeping. We need a mirror so I purchased the same mirror. I also purchased mirror mastic. I am afraid to hang it up without proper preparation of the area. The wall was painted about 3 months and i need to know if i have to sand down to the drywall or what? I really like the mirror and would like detailed simple directions on how to put this mirror onto my wall. we tried hanging the first one but could not get it to stop tilting.
Thanks in advance

 

Since you bought mirror mastic it should have directions for use. Keep in mind that some mirror mastic requires that the mirror have mechanical support at the bottom such as sitting on a vanity, clips or J molding. What is the size and thickness of the mirror?