Pulp starts with two ingredients ... cellulose (plant fiber) and water. We watercolorists are primarily interested in cotton as our source of cellulose for paper , so there's little reason to cover others.
Water should be free of impurities such as iron, and should also be neutral pH. If not, it must be corrected before mixing with the prepared cotton. Cotton should be comprised of the longer filaments of the cotton ball, called staple fiber. The most economical and ready source is from the garment industry where cutoffs from cotton clothing are plentiful. Otherwise, buying the raw cotton requires boiling and chemical balancing and washing, which is an industry unto itself.
The shredded cotton is tossed into a tub of water where a machine resembling a high tech paddle wheel, called a "Hollander," beats the daylights out of the mixture as it mashes and chafes the long strands of cotton fibers. This machine sounds like the engine rooms of a snorkel submarine ... it's deafening. The pounding procedure forces the water molecules deep into the anatomy of the fiber to initiate hydrogen bonding.
This is a crucial stage in the process of creating paper. Too much pounding will produce a brittle, translucent paper. Too little beating will render a very soft paper that might cause reworking problems for the painter.
When the pulp is ready, usually a synthetic internal sizing is prepared and added to the pulp. The stock is moved to vats and, in the case of handmade paper, the vat man “forms” the sheet or else it's piped to machines. Nearly all papermakers use the same internal sizing agent, but the surface sizing material varies greatly.