Defining a new slice

[Vittorio]

As a newbie I can create a new FreeBSD UFS partition/slice by means of 
sysinstall (by the way, I installed freeBSD 5.3).
Now recently I freed some partitions devoted to M$ Windows (at last!) and I 
want to transform them into UFS partitions.

How can I do it from a console?
Could you point me to an explanatory web-page I was unable to find googling?

A step by step explanation would be highly apptreciated.

Ciao
Vittorio