I've been making curtains for over 50 years from light sheers to lined and interlined.
Sometimes it's cheaper to make your own curtains, a lot depends on the fabrics you choose, and whether you can get fittings and accessories etc at reasonable prices.
If you're going to make floor to ceiling curtains consider whether you can physically cope with the size and weight of that much fabric, especially if you're lining them, and have you got a big enough space to spread the fabric out for pattern matching etc.
I never use the 'bag method' of making them, I always hand finish linings and hems. If you're using a fabric that frays a lot it's worth overlocking raw edges before you start assembling the curtains, they'll survive laundering better.
On the upside curtains are basically just straight sewing, there's just a lot of it. If the fabric is washable, wash it and square it up before you measure or cut anything. Most fabrics are not square even straight off the roll.
If you have to pattern match, don't have a seam down the centre of the curtain.
I always stitch the bottom hems before the tops, they hang better.
When you press the side seams and bottom hem, only press lightly, they shouldn't have a knife edge. I actually put lengths of batting down the side edges and bottom edge to keep a soft edge. I also use hem weights.
I've often been asked to make or alter curtains for friends or colleagues. It's always a bit worrying, so lots of double (triple?) checking measurements etc. Some years ago a lady asked me to put hidden pockets in the linings for their passports and valuables! I hope she remembered they were there before she took them to the cleaners!
A friend who sewed for Harrods showed me how to make some beautiful and unusual headings for curtains, so I made little mini curtains to practise them. They're now hanging in a dolls house!
I know this might sound like a lot of work but it's so satisfying to see your own beautifully made curtains gracing your windows. Good luck, let us know how it's going......