Right now keep breeding them. Soon you'll have around 100.
From those 100 in tank A, select the best 10 for your trait and move those that don't match your criteria over to tank B. Your "cull" tank.
Breed those in tank A some more, and then select again.
It can literally be that simple.
It can get lots more complicated, but especially in the beginning- keep the fun in it!
Welcome to my side of the world!