Taming cotton jersey takes a lot of time, but very little effort.
Lay washed cotton jersey out on a large table, folded in half -- lengthwise or crosswise as you find convenient.
Pat the grain straight, then baste all around the edges so that they can't curl. I used to pin them, but one day I ran out of pins, and realized that it's as easy to stick a needle as to stick a pin -- and thread doesn't have to be removed before you cut. Then spray with diluted starch. Hardware stores sell spray bottles intended for insecticide; these bottles are marked for diluting, and have very little inclination to clog up.
Allow the jersey to dry, then flip it over and spray the other side. When I want to be really, really sure, I give the fabric two or three coats of starch.
Oddly, I haven't found it necessary to wash out the starch before wearing the resulting garment -- even when I'm making underwear. YMMV