You can simply add your siRNAs to the liposome solution while vortexing. Electrostatic interaction of the siRNA with the cationic liposome should be sufficient for the formation of lipoplexes.
Gently vortex liposomes of homogeneous and controlled size with a diluted solution of siRNA. Wait for 30 minutes. Should be ready. Lipoplexes preparation should be nevertheless controlled by size and zeta potential, the latter allowing you to particularly verify the lipoplexes are formed according to their net charges.