Fairly easy to mix, you just need an insulin syringe or something that fits in the glass tube end (I used a pen). Insert it and gently push the stopper until it mixes in the glass vial end. The two stoppers should meet and push back (the vial is pressurized at this point) but if you get the stopper on the glass tube side only pushing back you may need to relieve the pressure in the glass vial end. Do this CAREFULLY with an insulin syringe with the vial right side up (should only release air) otherwise you'll spray very expensive GH all over.
That's fine, just keep pushing. There's a divot on the inside that allows it to pass by the other stopper. This is where the pressure release is important otherwise it'll shoot right back between the stoppers