Start by feeding your starter and wait for it to be bubbly and has doubled in size without falling back down.
Take a large baking sheet and line it with a silicone baking mat or with parchment paper.
Use a silicone spatula or pastry brush to spread out the starter as evenly and thinly as you can. The thinner you get it, the faster it will dry.
Place the pan in a turned off oven and crack the door with a wooden spoon. This will allow a safe space for it to dry that is slightly warm but still has air flow so it can dry. Remember to place a note on the oven so no one turns it on!
Allow it to sit until it is completely dry all the way through.
Break the dried starter into pieces and place in a clean container with an airtight lid. If desired you can place the flakes in a clean food processor and grind into more of a powder.
Label the container so you know what it is. Store in a cool, dark place until needed.
Notes
You MUST put a note on the oven to remind everyone in your household that your starter is in there! If you turn your oven on to preheat with the starter in there, it will bake the yeast and it will not be active anymore.