Making a tent isn't too hard, though it WILL be work.
You CAN buy canvas from Panther Primitives or other tentsmiths to use - the benefit of doing that is it is already water- and fire-proofed.
However, if you go down to the local home improvement store (Home Depot, Lowe's, etc.) and find a canvas painter's tarp, you'll be able to make a tent pretty easily and less expensively from that.
You will still have to waterproof the canvas, but a can of Thompson's Waterseal does wonders!!! Or if you want to go the more "period correct" route, linseed oil works very well. Neither of these will make the tent fireproof, so you will have to be careful with fires and candles.
We have also made "trail shelters" by using a tightly woven bed sheet - something along the lines of 300 threads per inch or more - and waterproofing them. They work great for treks, but they're not really big enough for camping for longer periods.
With a little effort, you can make a pretty simple but VERY effective shelter, called a diamond fly, by just hemming a square piece of canvas on all four sides. You will need to choose a size - usually we see them anywhere from 9' square to 12 or 16 feet.
We have a pretty large diamond - maybe 12' square - that our family of 5 used for a long time, especially when we were on "treks". Dad and I could carry it, but the girls couldn't, so if you are wanting to take it on a hike, make it a little smaller or use the lighter weight sheet idea. If you are just wanting it to camp in, then the larger size and a heavier canvas is definitely better, and you'll be able to keep all of your gear in it!
You will want to add some tie-down loops to the 4 corners and the centers of the 4 sides. A final loop right in the very center of the canvas will be great - IF you reinforce the inside and outside of that center with some scrap canvas. You CAN use pebbles or marbles tied into the corners, but it's not nearly as effective. If you want flaps, two triangles of fabric can be hemmed and sewn onto 2 adjacent sides and have loops/ties added. I've added a photo of a diamond with a sort-of sketch of what I'm trying to describe.
You set it up by first staking down all 3 corners of the main canvas. You might have to move them a little after you get it up, but that's not hard to do. Just be careful not to put them in deep at first.
Then put a pole with a peg on it thru the loop at (2) with a rope attached that pulls directly opposite the back corner. Stake that rope down.
Then you can either use a rope at the center loop (1) and thrown up over a tree or other support which will give you more uninterrupted space OR another, shorter, pole inside at the reinforced center, to give yourself some "lift". As you might be able to tell from the photo, if you use the inside pole, you can still attach a short rope to the center loop on the outside and stake it backwards to give it some stability.
A second pole can also be added towards the back if needed. I would suggest that if you think you might be doing this, add another reinforced patch at (3).
We've used ours all of these ways.
Here's another way to set up a diamond:
Finally, make sure all of your stakes are where you want them and set them deep.
The knot we use on our ropes is the tautline hitch, and if you haven't learned that one yet, here's a link to a Boy Scout page that shows it: http://www.troop7.org/Knots/Tautline.html
You can also use guylines with wooden blocks that keep them tight. The rope would have a loop in one end that goes over your tent pole or peg, or thru that center loop, and the other end goes thru a block that has 2 holes in it. It goes thru one hole, then up through the other with an overhand knot in the end. It makes a loop that can go around the tent peg. To adjust the tension, you just pull the block up or down the rope!
We have also used this tent with a fly out to the side to give some shade and cover should it rain. That's a different item all together. But it's easy to use too. It's really a square piece of canvas with loops or eyelets in the corners and at least the center of the sides. You can use poles to set it up or tie it between trees, or use it for a make-shift shelter, or a ground cloth, or throw it over gear that has to be left out, or........
For a ground cloth, you can use another piece of canvas that's been waterproofed, or that sheet I mentioned earlier. Either will work. And I'll tell you a secret.... SOME guys use a plastic tarp underneath a non-waterproofed canvas or rug. *grin* If we can't see it, we won't know its there! Just be careful to not have any part of your ground cloth sticking OUTSIDE of the tent. It will act as a rain catch and you'll be sleeping in a soggy bed.
That's it. That's all there is to building your own tent. Happy trekking!