Anyway here's a guide to changing/mending the screens (障子を張り替える) if you're nervous or curious.
First of all, shouji paper (washi) comes in several sizes and literally hundreds of types. You can get very high quality stuff, sometimes with beautiful designs (just like the screens themselves), or you can just buy the regular stuff in a supermarket for abour 300yen (it's reportedly possible to get paper-esque plastic, too). It comes in either squares for one panel/section, small rolls or rolls wide enough for the entire lattice (kumiko).
- You can buy special paper loosening liquid in home stores. Alternatively you can usually strip from the frame easily enough with water or steam. Moisten the glue lines and it should come off easily. If you just want to replace one panel then scour around the area and (if you're lucky) you should be able to peel it up and off without disurbing other panes.
NB: if someone has used the wrong type of glue or if it's been a particularly long time since the paper was changed, it can be very difficult to get off. Use a blunted scraper of some kind if you're left with soggy bits, but be careful not to destroy the lattice.
- When the frame is dry again, use shouji glue on the area that needs replacing. Buy the glue in a squeezy bottle in virtually any supermarket. These come with a handy nozzle which will apply the glue universally along the correct distance from the edge of the frame. Keep the glue line very thin (1-2mm).
If you're doing an entire door, it's fine to apply glue to the whole thing in one go.
- For a whole door, place the roll of paper at the bottom (tack or tape it to keep a straight line) and simply roll it gently down to the top. Some more expensive papers will have a smooth and a rougher side. The smooth side should face outwards. If your paper has a pattern, double-check to make sure your orientation is correct.
For one panel, row, use a small piece and roll or gently apply the paper.
Don't trim the edges until the glue has dried.
- Don't be tempted to use a cloth to smooth out the panels, as this can cause sagging if done too roughly. If you have any bad areas it may be because you used too much glue. In anycase, wait until the glue is dry and then reassess the situation.
Some people like to spray a fine mist of water onto the doors to tauten the paper.
- Trim off the excess. It's pretty frustrating to have half the paper catch and rip off when sliding across another door, so get this bit right.
Some people get very anal about their doors, and sometimes even want to make their own traditional rice-paste glue. If you want to know how, then here's a guide.
Tips/stories in the comments please!