So after getting a bunch of experience mending and altering clothes, I decided to have a go at making some things myself. This was my first attempt, a shirt, which I think turned out well. The fabric was some light curtain fabric that was lying about. Since I didn't want it to turn out too dressy, I used a pale green thread and left some of the seams on the outside. The buttons are some lovely vintage metal ones that came in a big bag someone gave me.
The way I did it was to copy a shirt I already had that I liked the cut of. I figured out all the pieces, and then traced each one onto paper by flattening the shirt on a surface such that each one at a time lay flat and drawing round the seams. I added seam allowances and then cut and stitched it together as close as I could to how the original seemed to be made. I've done a few more since, and once I've made a few more I'll try to do a full tutorial. Turns out, it's not so hard to tailor your own shirts!