There are some of the classical, dirty, routes: by means of dimehtylsulfate or methyl iodide. If you don't need a big amount they are usually the easiest ways. Basically you simple use a base and the methylating agent. Just be very careful and read the SDS.

There is a preparative in this paper:

http://pubs.acs.org/doi/pdf/10.1021/ja00153a012 (page 7).

And another one here:

http://www.sciencedirect.com/science/article/pii/S0040403998008855.

There is another way but a bit more complicated than using dimethyl sulfate or methyl iodide that is using dimethyl carbonate (DMC) under certain condition