Rinse 'em good inside and out and soak 'em with a few drops of vinegar in the water. Let soak a good half hour so they're not tough. I spray a little olive oil spray on my hand and slick up the stuffing tube then slide 'em on. Grind out a bit of meat first before pulling the casing over it and tying off to keep air out of the casing, then stuff away! If you soak out too much casing you can re-salt them and put them back in the fridge, they're good for up to a year. Just don't freeze 'em.
I have used a hand grinder and then pushed the casings on an antifreeze funnel. (new of course) I found I can push the sausage into the funnel and then ease the casing off the end as the sausage starts to fill it up. Daughter has helped and has a blast.