First part: Infinity stones

After doing the task you can find the stones in your power pack tab

Power stone: Basically just hit the orb on the ground really REALLY hard.
Time stone: Kill Dr. Strange
Reality stone: Kill Jane Foster
Mind stone: Kill Vision (Very hard to kill look in discussion comments for help)
Soul stone: Kill Gamora
Space stone: Same thing as power stone; just hit the teseract on the ground so hard

Second part: How to use infinity gauntlet

Equip the infinity gauntlet on a humans front arm, press the activate and the infinity gauntlet will forever be there, right click the infinity gauntlet to use powers, select a power and there.

In order to use the power simply just press the activate button (F for default) for punch, just punch him