My first book on bookbinding was Japanese Bookbinding by Kojiro Ikegami. One of the many useful things I learned from that book was how to back paper and cloth. I learned to strengthen fragile papers and make my own book cloth.
Over the years, I have modified the process. To back paper I use either plain cooked wheat starch paste or a starch paste/PVA mix. For better adhesion when backing cloth, I use a starch paste/PVA mix, with a PVA content of about 25%.
Lay the paper/cloth good side down on a hard smooth surface — I use my kitchen counter — and mist thoroughly with water.
Use a brayer to roll out excess moisture and flatten the paper/cloth thoroughly. Always work out from the middle, and be careful not to press creases into the material. The water in the paper helps to prevent the paste/PVA mix from soaking through and marking the good side of the paper.
Lay a sheet of backing paper out on another hard smooth surface. I use Chinese mulberry paper for backing — the kind sold in rolls for brush calligraphy — and I use a sheet of Plexiglas (Perspex) for the hard surface. (It looks brown in the picture because I have left the protective paper on the underside of the Plexiglas.)
Apply paste to the backing paper, always brushing out from the centre. If you are backing a synthetic fabric, you may wish to increase the amount of PVA in your mix to get better adhesion.
Lay the backing paper paste side down onto the paper/cloth. It is usually advisable to allow a margin of about 5 cm (a couple of inches), but I have been a bit stingy since paste on the back won’t matter for this project.
Using the brayer, smooth the backing paper onto the paper/cloth. You need to apply enough pressure to make sure the entire surface is attached, but not so much that all the starch paste/PVA is pressed out the edges. It is especially important to work out from middle with this step, as you don’t want to leave any air bubbles between the layers.
Apply paste to the outside edges of the backing paper. You need to cover a strip about 5 cm (2 inches) wide on all four edges.
Transfer the whole thing to the cleaned Plexiglas sheet with the pasted edges down and the good side of the paper/cloth facing up.
Use the brayer again to flatten the whole thing onto the Plexiglas. Again, work from the middle out to get rid of air bubbles. You will need to wipe the brayer clean after each roll to prevent paste getting on the surface of the paper/cloth. This is particularly important when working with dark materials, as every speck of paste will show when dry. I leave one corner of the backing paper sticking up, to help with removal later. Wipe off excess paste from around the edges.
Leave your backed sheet until it is thoroughly dry. Removing it before drying is complete will result in a cockled mess which you will only be able to flatten by re-wetting, re-rolling, and re-drying. When the paper/cloth is completely dry, you should be able to peel it carefully off the Plexiglas, starting at the corner you left unattached. If your proportion of PVA is quite high, you may need to use a pallet knife to ease things along. If everything has gone well, you will have a nicely backed, perfectly flat piece of paper/cloth for your project.