Simplest solution to record a screencast in Ubuntu

sudo apt-get install recordmydesktop

Start recording with 10 second delay with this command

recordmydesktop --width 1280 --height 720 --full-shots --fps 15 --no-sound --delay 10

Don't forget to adjust resolution first.

Note: In general, recording a screencast in Full HD is not very good idea. Most of people will view it on youtube and fonts will be too small to read without switching to full screen. I recommend reserving some fixed space on the screen for recording and adjusting recording position with --width, --height, -x and -y parameters. In my opinion using resolution 1280x720px is a good choice.

Reference

Usage: recordmydesktop [OPTIONS]^filename

Generic Options
  -h, --help                              Print this help and exit.
      --version                           Print program version and exit.
      --print-config                      Print info about options selected
                                          during compilation and exit.

Image Options
      --windowid=id_of_window             id of window to be recorded.
      --display=DISPLAY                   Display to connect to.
  -x, --x=N>=0                            Offset in x direction.
  -y, --x=N>=0                            Offset in y direction.
      --width=N>0                         Width of recorded window.
      --height=N>0                        Height of recorded window.
      --dummy-cursor=color                Color of the dummy cursor
                                          [black|white]
      --no-cursor                         Disable drawing of the cursor.
      --no-shared                         Disable usage of MIT-shared memory
                                          extension(Not Recommended!).
      --full-shots                        Take full screenshot at every
                                          frame(Not recomended!).
      --follow-mouse                      Makes the capture area follow the
                                          mouse cursor. Autoenables
                                          --full-shots.
      --quick-subsampling                 Do subsampling of the chroma planes
                                          by discarding, not averaging.
      --fps=N(number>0.0)                 A positive number denoting desired
                                          framerate.

Sound Options
      --channels=N                        A positive number denoting desired
                                          sound channels in recording.
      --freq=N                            A positive number denoting desired
                                          sound frequency.
      --buffer-size=N                     A positive number denoting the
                                          desired sound buffer size (in
                                          frames,when using ALSA or OSS)
      --ring-buffer-size=N                A float number denoting the desired
                                          ring buffer size (in seconds,when
                                          using JACK only).
      --device=SOUND_DEVICE               Sound device(default default).
      --use-jack=port1 port2... portn     Record audio from the specified list
                                          of space-separated jack ports.
      --no-sound                          Do not record sound.

Encoding Options
      --on-the-fly-encoding               Encode the audio-video data, while
                                          recording.
      --v_quality=n                       A number from 0 to 63 for desired
                                          encoded video quality(default 63).
      --v_bitrate=n                       A number from 0 to 2000000 for
                                          desired encoded video
                                          bitrate(default 0).
      --s_quality=n                       Desired audio quality(-1 to 10).

Misc Options
      --rescue=path_to_data               Encode data from a previous,
                                          crashed, session.
      --no-wm-check                       Do not try to detect the window
                                          manager(and set options according to
                                          it)
      --no-frame                          Don not show the frame that
                                          visualizes the recorded area.
      --pause-shortcut=MOD+KEY            Shortcut that will be used for
                                          (un)pausing (default Control+Mod1+p).
      --stop-shortcut=MOD+KEY             Shortcut that will be used to stop
                                          the recording (default
                                          Control+Mod1+s).
      --compress-cache                    Image data are cached with light
                                          compression.
      --workdir=DIR                       Location where a temporary directory
                                          will be created to hold project
                                          files(default $HOME).
      --delay=n[H|h|M|m]                  Number of secs(default),minutes or
                                          hours before capture starts(number
                                          can be float)
      --overwrite                         If there is already a file with the
                                          same name, delete it (default is to
                                          add a number postfix to the new one).
  -o, --output=filename                   Name of recorded video(default
                                          out.ogv).

    If no other options are specified, filename can be given without the -o switch.