Buy Greg Green's 'Cannabis Breeder's Bible'.

Don't use herms, buy seeds from solid, stable IBLs to breed with - pure Afghans, Skunk 1, Federation's Island Sweet Skunk, Matanuska Thunderfuck, etc if you can find these. Your males should be plants of good lineage and chosen for outstanding plant STRUCTURE and HARDINESS.

Your females, you get to choose what is most important to you.
Personally, I also look closely at plants structure and hardiness in my breeder girls, and then look at aroma, flowering time, and bud structure.

Good luck! It's a fun hobby.