We never had trouble mixing. 

Make sure it flows freely through the needle.  Once the manufacturer changed the needle and it was too small.  How horrible!

We used Emla cream one hour before and covered it with an occlusive wrap ( plastic wrap folded over and taped down with paper tape).  It numbed the spot and she never felt it.

We gave the shot after she went to bed. Her dad stood next to her to be sure she did not reflexively move her hands and hit my hand with the needle.  She never woke up during a shot.

We always gave it in the butt.  Is was easy.  Sometimes she would have a red patch at the injection site, but it didn’t last long.