Rita,
I quess we all have our own way of going about this. I like to inject, rub,then light up a allready set up WSM. When I reach the cooker temp I am looking for I put them on.
When I inject I have the injection liquid all over the butt"I pump them as full as I can get them". That is why I put the rub on last.
Good luck, let us know how it goes.
Rick