I would like to know a general procedure for the synthesising cis isomers from their trans counterparts and vice versa.
In general, any setup that gives enough energy to the compound can isomerize it. This can be done by heating it, irradiating it with visible or UV light, or by using a catalyst (reducing the energy required for transition rather than supplying more energy). Excitation of electrons from the $\pi$ to the $\pi^*$ (bonding orbital to anti-bonding orbital) allow free rotation about the $\sigma$ bond. Relaxation back to the $\pi$ orbital locks the configuration. The specific procedures depend on the compound of interest.
FYI, photoisomerization (light-induced isomerization) is the principle on which sight is based.
As a particular example of what buckminst is talking about, one common undergraduate organic chemistry laboratory experiment is the one where a tube full of dimethyl maleate is isomerized to dimethyl fumarate, with the help of sunlight and a teeny amount of bromine. Since solid dimethyl fumarate has low solubility in liquid dimethyl maleate, this makes for a nice visual demonstration. See e.g. this and this.