Practical Junk Rig by Hasler and McCloud is still the bible for flat cut sails. They have complete details and sewing instructions, and it is totally current for flat cut sails.
In my experience flat cut sails are fine in winds over five to seven knots but cambered sails are much superior in lighter winds. I think cambered sails still perform better in stronger winds but the differential is less.
All the best with the project. David.