It takes a little practice to stuff a casing, too full and they will split when you try to link them, not full enough and you get air pockets. If you get serious about making sausage I would recommend a proper stuffer it makes life a lot easier. Grind your meat, spice it, mix it and put it in the stuffer then fill the casings.
here's one of my stuffers yes I do have more than one.

Pork loins aren't the best cut for sausage they are too lean. A good juicy sausage should have 20-30% fat content. A pork shoulder (butt) is just about perfect as it contains around 25% fat. Keep the fat ratio in mind especially when adding venison to the mix which has no fat. If the fat contentis too low your sausages will taste mealy and dry.

ps. the Hypodermic needle is to pick air bubbles.