There are several methods for synthesizing siRNAs both in vitro and in vivo:

• Chemical Synthesis
• In Vitro Transcription
• In Vitro Transcription of long dsRNA followed by cleavage with Dicer/RNase III
• Expression from a DNA constructs such as a plasmid or viral vector

Chemical synthesis is the preferred and most widely used siRNA preparation method for transient experiments in cultured cells because siRNAs are easier to transfect than plasmids, and more importantly, because pre-designed, ready-to-use, gene specific siRNAs are available from Ambion (as Silencer siRNAs), making this by far the fastest and easiest method for researchers.

A table comparing the different synthesis methods can be found here.