It worked. I traced a pattern off an existing shirt, made a chart, and knitted it up. It's not beautiful, but it's really a sort of knitter's muslin. Proof that this will work in other (nicer) yarns and stitches.
Trace a pattern off an existing garment that fits you well. This is only likely to work with things that are made from a knitted fabric, unless you mean to knit at an incredibly firm gauge.
Lay the garment flat and inside out on a big piece of paper, and trace closely around it, holding down the seams as you draw along them, and making sure everything stays flat and doesn't move around. I used greaseproof baking paper - it's big and you can see through it. After tracing, fold the pattern in half, check that the two halves match, and make corrections. Repeat for each part of the garment. Sleeves are obviously more easily done folded in half.
I drew a line down the middle of each pattern piece, and along the bottom straight edge, and marked up the whole piece in a 1cm grid.
I smoothed out some of the curves, but others, like just under the armhole, or the neckline, I left exactly like the sewn seams, because I wanted to make something as close as possible to the shirt I had. This approach won't give you a very intuitive knitting pattern - it'll mean a lot of checking the chart closely. Instead of "Decrease 1 stitch every 6 rows" you're more likely to end up with "Decrease 1 each side, work 3 rows, decrease 2 each side, work 2 rows etc". So this is the time to decide how closely you'd like to stick to the original, and how much you'd prefer to flatten curves. Bearing in mind that knitting stretches, and looking at the simplicity of most pattern schematics, I'd guess that you could get away with quite a bit of line straightening. But I have yet to test that.
I knitted a swatch in the yarn I intended to use - a big swatch, to see how the yarn behaved on different needle sizes. I was aiming for a fabric with a drape and stretch close to the shirt's. Then I measured the swatch, and figured out my number of stitches and rows per centimetre.
Now it gets fiddly. I used knitter's graph paper from a book, but you can find it online. (It might make more sense to set up your own grid, at the same scale as the knitting. But that's not the plodding way.) I traced a grid off the graph paper that matched my gauge of 3 stitches and 4 rows per centimetre, and then copied the curved parts of the pattern to this grid. Putting the grid back over the graph paper, I drew stepped versions of the curves to match the stitches. Then I checked the numbers to make sure that the number of stitches and rows the decreases are worked over matched the lengths and stitch counts needed. A bit of fiddling, rubbing out and redrawing, was needed.
To make a chart I could follow while watching tv, I redrew the stepped curves onto regular graph paper. Just to make things simpler, I coloured the knit rows and left the purl ones blank. Much easier to check at a glance, and easier to make sure that decreases of more than 1 stitch were located at the beginning of rows.
I kept the full-size paper pattern handy while knitting, and every now and again checked my knitting against it.
I think it will be very easy to modify this pattern, to add length or width. For some reason I could never get my head around the calculations needed; now I can draw them directly onto the pattern, plod through a few fiddly bits, and have a new pattern.