how do i trim and start new plants off runners

HI Priscilla, Blackberries can be started by tip layering where new shoots in August are bent to the ground and covered with soil. over the fall and winter roots will form so in spring the new plants can be removed from the mother plant. here is a pretty good fact sheet about blackberries. http://www.ca.uky.edu/agc/pubs/ho/ho15/ho15.pdf