To make pulp, just tear cotton linters, or wood-based papers, or pulp packaging material (like egg cartons, but use only clean, unsoiled cartons) into small pieces, put them in a tall bucket, use a mixing attachment on your electric drill, and spin until as smooth as it will get. Then let soak for a few hours, and repeat the blending. This basically acts like a giant blender that separates the fibers without cutting them shorter.
Some people like to use a blender with a lot of water, one handful of pulp at a time, but it's tough on the motor, and it cuts the fibers even shorter, making it a weaker pulp.
Using a sieve, drain the pulp a bit, then finish with your hands to remove all the water that will come out. Store in a clean, airtight container in the fridge, for only a few days.
If you want a smoother result, or if you're not sure of the cleanliness of your pulp, boil it in water for a few minutes, let cool, then wring out and store in an airtight container in the fridge.
If you need to store your pulp for a long time, flatten it into sheets, let it dry fully on a clean towel and in front of a fan or in the sun (not directly on the ground or near bugs), then store as you would sheets of paper, in a container, away from dust.
When you need to use again, just break into smaller pieces, soak and blend. It will be faster than the first time.
For a smoother version, to use as a detail coat over a stronger pulp, you can break the pieces even smaller, and pulverize them into a powder, using a coffee grinder. Be sure to let the dust settle before opening it again.
I give credit where it's due, for this method, to David Osborne, in Great Britain.
I'm partial to cotton linters, for the strength of the paper it produces, and for the very simple way to prepare the pulp from those easy to decompose sheets.
I've gotten a sample ( a bucket) of wet cotton pulp from a paper mill, and it's bloody heavy to carry, on a bicycle. The same quantity, in dry form, would have been a breeze to carry, just like a sketchbook.
Still, cotton linters are not easily accessible to me, so I'm experimenting to see if the strength of boiled wheat paste, well-blended into newspaper pulp, will be enough for may puppetmaking and maskmaking needs and criterias.
See my preliminary results for pulp mixed with clay powder: