First of all welcome to the forum.
I would suggest that you don't attempt anything too fancy to begin with, just start simple, get the basics sorted out and then start experimenting.
Cut open the pork shoulder and estimate the amount of fat (%) that is present. If you can�t see any fat then their will still be approx 8 % fat present. I suggest that you aim for 25 % fat in your sausages, though some people prefer 30 %. Typical figures for fat are as follows
Lean Pork Shoulder 8.8 % fat #
Pork Shoulder (90 vl) 16.8 % fat #
Pork Shoulder (85 vl) 23.5 % fat #
Pork Shoulder (80 vl) 28.0 % fat
Shoulder Trimmings, Lean 18.65 % fat *
Shoulder Trimmings, Fat 41.05 % fat *
Pork Back Fat 78.6 % fat #
vl = visual/visible lean
* = after Moulton
For example
Assumed data
If you are planning to make 3 kg of sausage (25 % fat) you�re looking at a 750 g fat content.
10 % rusk/breadcrumbs
10 % water (iced)
3.0 % spice
77 % meat and fat (Pork Shoulder/90 vl)
Meat and Fat breakdown
1 725 g Pork Shoulder (90 vl)
585 g Hard Pork Back Fat
2 310 Total Meat and Fat Content (750 g fat content)
You can see from the figures above that if you are using a Pork Shoulder of 80 vl then you wouldn�t need to add any backfat as there is already a fat content of 28.0 %. In fact you may have to trim off some of the fat.
Method
1) Chill the meat and fat (if used) down to 1 �C (30 �F) prior to mincing/grinding.
2) Decide on what texture/coarseness of sausage you want to make
3) Cut the meat across the grain into strips approx. � inch to 1 inch (1.25 cm to 2.5 cm) wide. Make sure that all sinews etc. are removed otherwise they will clog the plate and you will have to remove the plate and clean it out.
4) Cut the fat (if used) into small pieces approximately � inch to 1 inch (1.25 cm to 2.5 cm) square. MAKE SURE THAT ALL SKIN, GRISTLE HAS BEEN REMOVED.
5) Run/pass the meat and fat through the mincer/grinder the first time into a meat lug or similar.
6) Mix spices (for applicable recipe) thoroughly together and sprinkle EVENLY over the surface of the meat mixture. DO NOT MIX IN YET.
7) Immediately after add the rusk, again evenly distributed, and lightly though thoroughly hand mix into the meat/fat mixture. Use rubber gloves if you want.
8) Gradually add the iced water and thoroughly though again lightly mix in until the mixture feels moist and slightly sticky.
9) Change mincer/grinder plate (if necessary)
10) Add sausage attachment and place rinsed casing onto end of stuffing nozzle making sure that it is not all pushed to the back of the nozzle. Typically for a normal sized sausage, 1 metre (3 ft) of 32 mm skin is required per 500 g (1 lb) of sausage.
11) Run/pass the mixture through the mincer/grinder the second time. You need to use two hands to do this, one supporting the sausage as it comes off of the nozzle and the other to feed the casing forward. DO NOT OVERFILL THE CASINGS, only practice will get this part right. Take care that the filled casings are not left to hang unsupported from the nozzle as the weight can weaken the casing causing it to burst whilst linking. When all of the meat/fat mixture has been added to the grinder put in a piece of white bread into the mincer/grinder, this will force out the remaining meat stuck along the spindle.
AS YOU CAN SEE MAKING SAUSAGES ON A MACHINE LIKE THIS REALLY TAKES TWO PEOPLE
12) Take the filled casing into your hands and using the thumb and index finger of your right hand lightly squeeze the meat out of the casing at the length that your want your sausages to be. Twist the first link away from you then repeat the process twisting the next link towards you. Continue until all of the casing/sausage meat is used up.
13) Allow to stand (on a wire rack it possible) at room temperature for approx 20 minutes to allow the casings to dry.
14) Store covered at between 1 to 4 �C (35 to 40 �F )
Hope that this is of some use to you
Kind regards
Parson Snows