A method comprising: disposing a sidetracking assembly within a cased wellbore, the sidetracking assembly fluidically coupled to a drill string configured to receive fluid from or near a surface of the wellbore, the drill string attached to a milling system, the sidetracking assembly comprising a cement retainer and a whipstock assembly releasably and fluidically coupled to the cement retainer and disposed between the cement retainer and the drill string, the whipstock assembly comprising an anchor activable by fluidic pressure and the cement retainer comprising a packer activable by fluidic pressure; deploying a first ball inside the drill string, the first ball configured to leave the drill string to move past a ball seat of the whipstock assembly to land on a ball seat of the cement retainer to block a first fluid outlet of the cement retainer; with the fluid outlet of the cement retainer blocked, setting the cement retainer in the cased wellbore by applying fluidic pressure at the cement retainer to expand the packer; exposing a second fluid outlet of the cement retainer to fluidically couple the drill string to a portion of the cased wellbore downstream of the cement retainer; flowing cement through the drill string, to the portion of the cased wellbore downstream of the cement retainer to plug the portion of the wellbore; separating the whipstock assembly from the cement retainer; deploying a second ball inside the drill string, the second ball comprising a diameter larger than a diameter of the first ball such that the second ball lands on the seat of the whipstock assembly to block a fluid outlet of the whipstock assembly; with the fluid outlet of the whipstock assembly blocked, anchoring the whipstock assembly in the cased wellbore by applying fluidic pressure at the whipstock assembly to expand the anchor of the whipstock assembly; and with the whipstock assembly anchored on the cased wellbore, actuating the milling system to begin cutting a sidetrack window in the cased wellbore. disposing a sidetracking assembly within a cased wellbore, the sidetracking assembly fluidically coupled to a drill string configured to receive fluid from or near a surface of the wellbore, the drill string attached to a milling system, the sidetracking assembly comprising a cement retainer and a whipstock assembly releasably and fluidically coupled to the cement retainer and disposed between the cement retainer and the drill string, the whipstock assembly comprising an anchor activable by fluidic pressure and the cement retainer comprising a packer activable by fluidic pressure; deploying a first ball inside the drill string, the first ball configured to leave the drill string to move past a ball seat of the whipstock assembly to land on a ball seat of the cement retainer to block a first fluid outlet of the cement retainer; with the fluid outlet of the cement retainer blocked, setting the cement retainer in the cased wellbore by applying fluidic pressure at the cement retainer to expand the packer; exposing a second fluid outlet of the cement retainer to fluidically couple the drill string to a portion of the cased wellbore downstream of the cement retainer; flowing cement through the drill string, to the portion of the cased wellbore downstream of the cement retainer to plug the portion of the wellbore; separating the whipstock assembly from the cement retainer; deploying a second ball inside the drill string, the second ball comprising a diameter larger than a diameter of the first ball such that the second ball lands on the seat of the whipstock assembly to block a fluid outlet of the whipstock assembly; with the fluid outlet of the whipstock assembly blocked, anchoring the whipstock assembly in the cased wellbore by applying fluidic pressure at the whipstock assembly to expand the anchor of the whipstock assembly; and with the whipstock assembly anchored on the cased wellbore, actuating the milling system to begin cutting a sidetrack window in the cased wellbore.