It depends what you mean by 'a synth'.
If you just want to get some sound out of Jasuto:
1. Tap [Create] -> [IO] -> [Speaker]
2. Tap [Create] -> [Synth] -> [Sin]
or [Saw] etc
3. Drag the Speaker under the waveform and you should have sound.
To make more complex synths:
1. Do the above (Add a speaker and a waveform)
2. Add any number of other waveforms, effects, signal processors etc. via the [Create] menu.
3. Move the added elements around to create different sounds
For more in-depth information, see
Basics:
http://www.jasuto.com/site/?page_id=61Details:
http://www.jasuto.com/site/?page_id=110Hope that helps,
- PJ
P.S. To see how to use Keys or Seq, see the first two patches on the video at
http://www.youtube.com/watch?v=uHlrzIrimO4