To inject the meat you typically take the brine and dilute it by about half then inject the meat with 10% of the weight of the piece of meat with this diluted brine. All you are doing is adding some insurance to be sure the cure penetrates the thickest sections of the meat. If it doesn't, no harm done you'll just have a coin sized piece of uncured meat in the middle of the loin. You just want to slowly inject it to the point where it is beginning to come out the hole and your goal is to get the cure into the thickest section. You really need a brining needle for this. They sell them in the garden section at Home Depot and places like that.
I don't know the recipe you are using so its hard to say much more but one thing you can do without injecting or with injecting is when you take it out the brine, rinse it good then put it in the fridge for a day or two longer. This will allow the cure more time to equalize and pass through all the meat. In other words, be patient and don't get in a hurry and things will work out better in the long run. Nothing can pizz you off more than seeing one of those corn shapes in the middle of the meat.
