First principal is don't over-do it. Start with doses that don't provoke an intolerable detox/cleansing reaction. If you have a headache, nausea, or diarrhea, back off on doses, or skip if needed.
Don't drink coffee or alcohol while the MMS is in your system. Do drink plenty of water - at least 8 oz/240 ml of liquid in or with each dose. Don't take any vitamin C, or use juices with added vitamin C while taking doses. If you're doing a longer course of MMS, I strongly suggest 1000 mg of vitamin C 2 hours after the last dose of the day, maybe more, and get good nutrition including any supplements which I'd recommend taking after the MMS for the day and before bed.
Above all, be observant of what you're experiencing and use common sense. MMS and any wellness modalities are part of taking self-responsibility for your own health - literally, you're the "doctor."