Reverse steps for replacing front panel glass

I used ifixit guide for this glass replacement after purchasing a replacement glass and tools from ifixit. My new glass however keeps sliding down when the MacBook Pro is open, i.e. with the screen in the upright position. I did not apply the heat gun in the reverse of step 17.

My questions are

1. Should I apply the heatgun to get the glue to take properly.

2. If yes to 1, so I need to disassemble the MacBook Pro to do this, or can I just do it on the fully assembled unit.

Thanks

Mike