Before placing your sourdough starter into the fridge, you need to feed it. I recommend feeding your starter with a 1:2:2 ratio before you place it into the fridge. This just gives it a little more food and ensures it won't go hungry.
Take 25g of sourdough starter and place into a clean jar. Add 50g of water and 50g of flour. Give it a really good stir to ensure that it is thoroughly mixed through.
Place the lid on the jar and screw in place.
Place the jar of starter at the back of the fridge (where it's cold) and leave it there for as long as you need to. It's best if you feed it at least every 2 weeks (if even if you're not using it). However it will survive for quite a while without feeding.