With a sharp knife, poke 2 or 3 slits into each serrano pepper and pack them into a quart sized mason jar. The slits make sure that the liquid in the brine will get inside the peppers, so make sure they're cut all the way into the middle.

Roughly chop garlic cloves and add them to the jar with the peppers. Try to pack them down a little. 

Add vinegar, water, sugar, salt, and msg (if using) to a small saucepan over medium-high heat. 

Bring the vinegar mixture to a boil. As soon as it starts boiling, stir mixture, remove from heat and pour it into the jar over the peppers. This is obviously a hot pot pouring hot liquid, so be careful. I typically do the pouring process in the sink so any spilled near-boiling liquid does not hurt anyone. 

Close the jar lid and let it rest on your counter until cool. 

Add to the fridge and enjoy some great pickled peppers over the next few weeks.