I've known several commercial seamstresses, and it is possible to make curtains starting by making your own pattern. Your local sewing center may even have a few books. Of course the wrap around windows defy the old rectangular piece of fabric rule... so here goes.
If you want to do formal tucked drapes... talk yourself out of it, it's a pain in the ****, skip right down to the next section. So here's the PIA tucked method:
Basic rule to buy fabric for tucked drapes. To make a finished panel 30 inches wide, you'll need to start with fabric that is 90 to 120 inches wide - which means seaming the fabric pieces together.
To make your pattern, get some heavy and wide butcher paper or brown wrapping paper and tape it over the windows leaving whatever margin you think correct to fit over the top (and if you have them bottom rails). Allow some overlap on each side too. I use masking tape because it leaves no residue if you remove it promptly. The pieces that go over the sied wrap arounds will actually have a shape vaguely like a slice of angel food cake - wider at the bottom, with a slight inward curve on the top and a slight outward curve on the bottom. Make your marks with a pencil, then cut out - and you'll have the size and shape of the finished panel. Add 1.5 inches to each side to fold over, finish and attach through the lining onto the curtain. on the curved pie shape, it's difficult to do a fold over, so I add a 5/8 inch seam allowance and cut a strip 4 5/8 inch wide for the top foldover, then sew it on. Decide how wide you want your tucks to be and how deep (use the top allowance - it needs to come out evenly so if the finished length will be 30 inches, you'll want a number of tucks that divides evenly into that - at 3 inches can have 8 tucks and leave three inches plain on each end. Figure your fabric used for each tuck - if I choose a three fold tuck that's 5/8" deep it will take 5/8" it will take a pinch over 3 1/2 inches of fabric per tuck. Add that to the width of each panel for each tuck. The bottom edge of the wrap around panel will be about 8 inches longer than the top. To put in the tucks you'll have to space them further apart but keep the tucks the same depth. (I did my first one on a cheap old sheet as a test.).....
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NOW SOBER UP AND TRY something easier. IGNORE the fact that the side windows are trapezoidal - make a rectangular curtain based on the bottom measurement and once on the curtain carriers let the top be a little more gathered than the bottom. If you're using a plaid fabric it will actually look better if you just make the panel rectangular.
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ALTERNATIVE #3 - even easier and with the right fabric choice very attractive - Gathered drapes. Not suitable for heavy fabrics like velvet, but works fine for lighter weight fabrics even with a lining. You'll possibly have to look in K-mart for cheap spring rods. You may have noticed that sheer curtains placed over doors with windows or sidelights to those doors are often secured top and bottom with a spring instead of a rod. The flexible spring hooks over two small flat hooks at each edge.
Make rectangular curtains 3 to 4 times wider than the window you want to cover and leave a pocket to gather them up (threading the spring through the pocket). Most people leave a 1 to 2 inch ruffle above the pocket - attractive and prevents the edges of the curtain from fraying due to stress from the rod. Works great on a flat window. On the curved wrap around windows, you COULD use this same idea if you have top and bottom curtain carriers. Just hand sew the little plastic curtain carriers to the back of the pocket about every 4 inches. Slide the plastic pieces into the slots, then stretch the spring and fasten to screws at each end of the window.
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There is another way to make gathered drapes - permanently on a flat backing piece - but the previous way lets the gathers come out when you want to launder and iron them.
Hope that helps.