The magic behind treated papers and wet-shaping complex models. Methyl Cellulose (MC) strengthens paper fibers which often gives the "crispy" trait to desirable origami paper. This bottle has MC in powder form. I normally fill a regular plastic cup 75% with water and insert a large spoonful of MC.
Paint Rollers have been the best tool for applying methyl cellulose on paper in my experience. They make much more even layers and won't tear the paper as easily as brushes.
Your best friend when cutting out squares. The sharper the knife, the better the cut, and this knife is super sharp. Be careful when using this tool and be responsible. If you are under 13 years old, ask your parents before using these.
Cutting out a perfect square from treated papers can be a hassle. A metal ruler is a must-have for making this process easier. It is both safer and more reliable as you can even tape down the ruler to keep it from shifting. Wooden rulers would wear out from that process.
This will be one of the best investments in origami you can make. Having a reliable surface to both treat and cut papers on is such a blessing. I like to even treat papers and leave it stuck to the surface to make cutting perfect squares extremely easy. Blick Art Materials has an amazing deal on this size
Giant Cutting Mat
This is the same as the other cutting mat but much larger. While the price is very steep, it will offer you much more flexibility to treat large papers like Wenzhou Rice paper. If you are thinking about making Ryujin sized papers, this will be what you want.