I have a blouse I like. I like it a lot. It's nothing special, just a short sleeved blouse with no collar. I wore it recently and decided I liked it enough to copy it. So, here's what I did:
I got out a roll of Christmas wrapping paper. It's the only paper I had around and I'm too cheap to go out and buy any.
I analyzed the blouse to see its components - back (cut on fold), sleeves, and front. To make the facing, I'd just duplicate the neckline to the shouder and the front and make a facing just like I'd make for a storebought pattern. Easy.
I laid the blouse on the paper, starting with the back which I would eventually cut on the fold. I didn't take the blouse apart, but I was very generous with my use of pins while putting it on the paper. I spread out the blouse to avoid sizing problems with hidden folds, the neck, the shoulder, and the armhole. Then, I drew a line around the pattern piece, carefully duplicating the curve of the neckline and the curve of the armhole. After I unpinned the blouse from the paper, I marked the pattern as Back and noted the Cut on Fold section. I planned on adding seam allowances when I actually cut the fabric.
Next came the front. This was a little tricker. I repeated the steps above, taking care to respect the lines of the collar, armhole, and shoulder. Those needed more attention than the body of the blouse as they were sewn to other parts and needed to be gently hand pressed as I went along. Again, I used plenty of pins and drew the lines, cutting and then marking the pattern piece.
The sleeve was pretty easy. Lots of pins along the curve of the armhole. This sleeve happens to be symmetric, so I made the pattern piece such that I could cut it on the fold (the top seamline - there's no seam there - made a great fold). Pins, pencil, cut.
I waited to cut facings until I cut the fabric itself.
I pinned the pattern pieces on the fabric. For the back, I put an extra folded piece of fabric under the folded fabric that would be later used as a facing. I pinned the pattern on and cut the back and facing at the same time. I decided to trim that facing down later.
For the front, I placed a couple of fabric pieces under the body of the blouse where the front closures would go and cut that as facing along with the cutting of the blouse. Again, I decided to trim the facing later.
I stitched the shoudlers of front facing and back facing, right sides together, and serged the seams. At this point, I have an exact duplicate of the blouse cutting with the exception that the edge that doesn't match the front edge and neck edge of the blouse are sort of not cut perfectly. I used my serger on what would be the finished edge of the facing to finish and give me a seam allowance to turn under and stitch. This is when I trimmed the facing .
I stitched the shoudlers of the blouse front and back and then stay stitched from center front around the blouse and back to the center front on the other side.
I pinned the facing to the blouse, right sides together and then stitched the bottom to the bottom, all the way around. Then I turned this right sides out and pressed the facing down, stitching the shoulder facing to the shoudler seam inside to hold it in place.
From here, I added the sleeves as I would any other sleeve, hemmed the blouse and sleeves, added buttonholes and buttons, and voila. A blouse that fits well, looks good, and cost just about nothing to make!