How to close the iPhone 6 properly

When replacing an iPhone 6 screen i have the following thing that botters me.

The iPhone on which i replaced the screen has a little 'space on the upper left side (on the height of the volume buttons) when its dark you see a little light. also if i push a little harder the screen goes a tiny bit down.

Question: Does this have to do with the way i click the screen in the phone?

Instruction video A shows: you have to click from top to bottom

instructoin vido B shows: once you placed it on the top, first click the bottem in and them slowly click the screen in upwards....

any advice?

Thanks