I decided to start with the simple style familiar to us old hippies - completely rectangular construction, embroidered yoke and short sleeves. Since it's summer, some nice light cotton gauze or linen is fabric of choice.
with a couple yards of fabric - the rectangular construction wastes
practically nothing. The embroidered bits are marked out and worked before construction.
Many Mexican blouses have the yoke the same size/depth front and back - I raised the back of mine (this helps keep the blouse from suddenly becoming an "off the shoulder" model) and lengthened the back panel accordingly.
Finally, decide how "deep" an armhole you need - remember there will be a 5-6 inch square gusset inset, so about the depth of the sleeve on your favorite-fitting T is a good place to start for drafting purposes.
( Just for informational purposes, I like a "loose" women's size 26 blouse; my pattern pieces came out as follows - front panel 20"L x 36"w, back panel 27"L x 36"W, horizontal yoke pieces 3" x 15", yoke straps 3" x 7", sleeves 20" x 8", gussets 6" square (add your desired seam allowance to this). Your mileage will vary with your own measurements... )
I have seen vintage blouses where the fabric of the front/back panels is prettily finished at the top, then cartridge-pleated to the yoke so that the top edge forms a pretty little ruffle - I'm thinking I'm not going to be that fancy, but if you do it please send me inspiring pictures!
So the pattern is worked out, mockup fits, fashion fabric is on hand, next we go to the real thing.