http://monerotoruzizulg5ttgat2emf4d6fbmiea25detrmmy7erypseyteyd.onion/resources/user-guides/multisig-messaging-system.html
Doing so is easiest on a Linux system. With the next hardfork in Spring 2019 the MMS will become an integral standard part of the Monero software: You install Monero, you have it. A word of caution: At the time of writing using the latest development Monero version does not lead to conflicts and complications with any regular Monero release software and downloaded blockchain on the same system, but that may change between now and the hardfork, especially near the hardfork.