There are many proper ways to cat scratch, but I'll give you my version now. What I do first is to remove the debris like sticks, pine cones, etc.. off to the side. We'll use this to naturalize the area when we are done. I also save the top soil in a little seperate pile too, cause as you dig, you'll see there are layers in the dirt we want to preserve. I then scratch a hole using a stick, shovel, sharp rock, sheath knife, or ice ax. I make my hole at least 6 inches deep, and as big around as you need. (somewhere around 6-8 inches in diameter for me).
Once I've deposited the paper in the whole as I get done, I *always* burn the paper. This is for 2 reasons. One, paper can take an amazingly long time to decompose, and the look of white "paper flowers" is ugly, and can be seen from a distance, ruining ones senic view. The other reason is the small amount of ashes will help limit the fly->shit->food->me connection. I always carry a cheap Bix lighter, so I can burn the paper. Once the paper is done burning, it's best to then pee directly on the ashes and the pile. This not only to make sure the ashes are tottally out, but it helps start breaking down the shit, and also peeing on it can make a recognizable pile look much like the rest of the dirt. And btw, a sister taught me that trick, so there is no excuse to not learn this useful technique.
Once this is done, I then replace the dirt, then the topsoil, and then I naturalize it with the debris I placed off to the side. While this process may seem time consuming, it is not. Both my kids learned how to do this when they were very little... To shit properly in the woods is the best way to not spread contagious disease to your friends, and a good way to help keep our impact on the earth light.
Slit latrines were pretty much defined by the Army Corp of Engineers, who have a lot of practice at this too, to be 6 feet long, a foot wide, and several feet deep. Not that we need to measure our shitters to be army regulations, but you get the general idea. It should be as deep as you can sensibly make it, and not too wide, so children and us short people can squat over it without falling in (which I've done, and it is no fun).
All good shitters should have paper and hand wash. It does rain and snow, so the paper should be stored in a waterproof bag, or a coffee can. Wet toilet paper in the rain is no fun... Handwash is also a good thing, because washing ones hands after shitting is a great way to not spread disease. Handwash works good in 1 gallon containers with a *tiny* bit of bleach per gallon. Fancier setups have the water rigged so you can use foot levers to turn it on, but I'm always glad to at the least find an old milk jug.
Over the years I've seen all sorts of shitter covers. It's best to
cover your shitter, to keep flies out. One of my favorites was in
Arizona, which was the deluxe box style.
The box style ones aren't as popular as I
would like due to you've got to make the box, but for people that have
problems squatting, for whatever health reason, they are nice to
have. This particular one also never ran out of toilet paper or
handwash the while time I was there, which was nice.
A more common variation on the box style, is this one
from New Mexico. 
This size is more easily built, and
transported to the site. This one had a really nice location in a
secluded spot, but near a main trail. If you don't have time to make a
fancy cover, the long-time technique is to cut a hole in a sheet of
plywood, and make a hinged flat on the board. This can also be made
with boards and and tarp. The main idea is to keep the flies out.
If you have no shitter cover, you can then put a mix of fire ashes and
if you have it, quick lime. Just fire ashes work ok, or a very thin
layer of quick lime. If you have agricultural lime instead, this is
much stronger, and it must be cut with ashes at about 1
tablepoon per 5 gallon bucket. Flies won't go through ashesi or
lime. If there are no ashes or quick lime by the shitter, about all
you can do is scoop some dirt from the mound by the shitter, and cover
things best you can, and hope somebody comes by with ashes later. The
problem with just using dirt, is that flies will dig through an aweful
lot of dirt to get your shit, so it's not really that effective. But
hey, anything helps.
When your shitter is full (10-12 inches from the top), it's time to cap it off. First, make sure you have quick lime, not agricultural lime. Too much agricultural lime will screw up the PH of the soil for a long time, which will effect the plant growth somewhat, depending on the ecosystem. It is very strong, and must be cut with ashes before using.If all you have is agricultural lime, then it's better to use no lime, than too much. With quick lime, make a thin layer to cap it off.
To cap it off with dirt, make a pretty good sized mound, cause it'll settle. A level cap of dirt will leave a depression, as things settle. It's pretty tough to pack the dirt as tight as it can be. The mound of dirt should be then covered with logs, sticks, and other local material until it is naturalized.
In the desert, or where it is hard to dig a hole, it's usually best to put the paper in a bag, and burn it.
I've found the best tool for digging shitters to be a digging bar. A digging bar is a 5 or 6 foot length of inch thick iron bar, with a point on one end. I use this to loosen the dirt, and then a shovel to remove the dirt. This is much faster and easier than digging with a shovel, and it's even better than a pickaxe. Plus, a digging bar is great for removing the rocks you inevitably will find. Ideally you should pile your dirt on a tarp and cover it, so you have some when you need to cap off the shitter. Sometimes, it you don't do something with the dirt, it can wash away, making it difficult to have enough dirt to cap it.