About 20 pounds of apples = 1 gallon of juice. Work from there.
The apples need to be cleaned in water or a water/Potassium Meta BiSulphite mix - Apples sometimes have a lot of wild yeasts on them depending on how the weather has been and how long they have been sitting around.
Then quarter them, remove the seeds (nasty things), and mash up as much as you can - potato masher, blender, food processor; appleas are a bit tougher than soft fruit and need to be pressed as much as possible. The best is to use an apple press and get just juice.
Make sure you use pectic enzyme, it will help clear the cyser.
The problem of course is you have 20 pounds of apple mush, rather than a gallon of juice. You'll have to add water, and then it's not really a cyser, it's more of a melomel...