I purchased some sharpening stones from Nihonzashi to try out. They have a fairly large selection of stones from 100 to 8000 grit. This is based on the Japanese grit system, and all the stones are from Japan I believe. I only purchased a 1000/6000 combo stone, and a 3000 grit one, which are better for maintenance not hard core repair. The Japanese grit system is not the same as the US one (quite different actually). There is a conversion chart that I saw somewhere but you will need to be careful when selecting them. I tried them out on the class sword (Practical XL Light).
Just some basics: Do NOT try to sharpen your sword if you have never done it before, especially if it is an expensive or old one. I'm okay with trying it out on our $300 class sword which is pretty scratched up as it is. The following is also NOT to be used as self-guided instruction. You will still need your sensei or a properly trained person to instruct you.
To prepare the area:
The sharpening stones I have are about 8 inches long and 2 inches thick. You basically place it in a stone holder (basically adds a frame with rubber feet) and place it on a table height (if sitting), and you'd slide the blade over it. Other stones, not these ones, are smaller finger sized where you keep the blade in place and rub the stone on them. Make sure that you have room to maneuver the full sword all around meaning you need at least 3 feet radius all around you. Best if you face your back to the wall so no one comes up behind you (safety fact). I did mine sitting on a chair, but you can do it standing too. Traditionally, it would sitting on the floor but that just hurts my back after a few hours.
To prepare the stones:
These are wet or water stones. Basically, each needs to be soaked in water for 15 minutes and kept wet while sharpening. A little baking soda helps to balance the water so that the sword does not get rust stains. It will get salt stains but those was off easily.
To prepare your sword:
If you know how (and you should), carefully remove the mekugi and then the tsuka and all the fittings until you have the naked blade itself. You'll need one or two dish towels to tightly wrap around the blade; it's important that it is tight, because the blade cuts most when it slides on an object. Even the nakago (tang inside the handle) can be sharp even when not blade-sharp, so keep that wrapped. Wipe off all the oil on the blade with a cloth, until generally dry.
The sharpening process is generally a slow painstaking task that has to be repeated at constant speed and pressure. I typically rub each side of the sword on stroke up and then down, and repeat that 100 times. Then I flip over to the other side and do the same. This has to be repeated for each of the stones. Basically, you'd start with the lower number grits (the coarser ones), sharpen both sides 100 times each, and then move to the next higher stone, repeat, and so on. Each stroke takes me about 4 - 6 seconds (which is slow when you actually measure it out).
100 strokes x 2 sides x 3 stones = 600 strokes. x 5 secs = 50 mins + some breaks switching stones = 1hr 15mins.
The speed and the pressure is particularly important. First of all you don't want to press hard. It's not about rough pressure and scraping the surface. This is hard to describe even in person, which is why this takes so much skill (even way beyond mine) to do professionally. I hold the sword wrapped around the nakago (tang), and use the fingers of the other hand near the monouchi (cutting section) of the sword to press down. The sword itself is not a flat object and you need to hold it at a very slight angle to sharpen to the edge; otherwise you're just polishing the side and not sharpening at all. Throughout the entire process, you will need to periodically re-wet the stones by sprinkling water (with baking soda), sometimes wiping the dirt of the sword with another cloth.
Finally, cleaning, and putting together the sword:
The sword will have salt and dirt on it so you need to wash it off. You can use the baking soda-water preferrably, or run under a faucet, or soapy water. Quickly wipe dry immediately after. You want to quickly inspect it. Do not rub your finger along the edge, but you can test it by running 90 degrees across the edge (scraping your finger tip essentially). This is still dangerous so do not put pressure, and keep the blade on a table top. Wipe the sides with Choji oil as soon as you can.
Once you're ready, put the fittings back on the sword in the same order they came out in. Otherwise they will not fit right. Put the handle back on. Now the trick is to get it fitted tightly. The traditional way is to hold the sword completely vertical in a hammer grip, then use the palm of your other hand and tap the bottom of the sword several dozen times. This moves the tang deeper into the handle itself. Your palm will likely hurt. The way I use isn't good if you have expensive fittings: I use a rubber coated mallet to tap the bottom rather than my hand. It's the same effect, but start with soft taps and increase pressure only if nothing is moving. When fitted properly, there should be no gaps or rattling of the fittings. When its in there tightly, you will need to use your tiny sword hammer to tap the mekugi back in place. The mekugi needs to go all the way back in (not sticking up at all).
Inspect the sword again, and again wipe some choji oil on both sides before you put it away.
It may take about 5-15 minutes to take apart your sword, and another 15-20 to put it together. So all in all, this will take you at least 2hrs to 2hrs 30 mins.
It's a complicated process that you really have to see for yourself but worth it. We tried cutting with the class sword yesterday and for me it is finally coming up to par as an okay cutting sword.