I'm assuming here that you know the basics of hemming and finishing.
What I was taught when making curtains is to measure carefully and make perfect, right-angled rectangles of fabric. This assumes, of course, that we live in a world where all buildings are square and straight and true.
This may be how it is when they're first built, but over time, gravity takes hold. Things shift and settle and warp (I am just talking about buildings, right? I mean, that won't happen to me, will it?). Hanging your perfect rectangle of fabric in a window or doorway of an old building will make it look like you don't know the first thing about sewing. Here's what I do.
I take the careful measuring and sewing process to the point that I have a curtain that's the width I want it and several inches longer than I need it to be. My favourite hardware for hanging curtains are those metal rings with clips attached and copper plumbing pipe for the rod.
I put the curtain rings on the curtain, slide it onto the curtain rod and hold it up to my window. Sometimes, hanging the rod evenly on the window will give you a curtain that gaps or just doesn't hang straight. You might have to fudge it and hope no one notices.
Once the rod is where you need it to be, you can measure for the length of the curtain. I like a nice hem on the bottom (I make a lot of my curtains out of old sheets and use the existing hem for the bottom). I line this up where on want it - along the window sill, say, or, on a closet, the floor. Adjustments in length take place at the top of the curtain, where it hangs from the rings (this can be a time-consuming, labourious process, but it's worth it. If you don't take the time, your curtains will bug you forever).
Once you have the curtains to the size and shape you want and the bottom is hanging nice and straight, you can hem and finish the top.
It's not quite how they taught us in Home-Ec, but it works.