  Practical aspects of the robust method we recently proposed for producing
few-cycle attosecond pulses with arbitrary waveform in the extreme ultraviolet
spectral range are studied numerically. It is based on the undulator radiation
of relativistic ultrathin electron layers produced by inverse free-electron
laser process. Optimal conditions for nanobunching are given; attosecond pulse
energy and waveform, and their stability are studied. For K=0.8 undulator
parameter, carrier-envelope-phase stable pulses with >45 nJ energy and 80 as
duration at 20 nm, and >250 nJ energy and 240 as duration at 60 nm are
predicted with 31 mrad and 13 mrad phase stability, respectively.
