question about serializing task

Hi,
I am using MrProject for the first time and I have a problem about
serializing tasks: I have many tasks that should be accomplished by the
same person and, of course, these tasks should be serialized. I do not
have any preference about the order of these activities, so that any
order is good. I do need the serialization since I need an estimation of
the duration af the project.

Actually MrProject let all activities start the first day of the project
but that person will not be able to do all of that at the same time :-)

How do I set this kind of things in MrProject?

Version information: mrproject 0.9.1 on Debian/powerpc "testing".

I thank you very much,
Giuseppe Sacco