Keeping a Leaven (the short version)
A leaven is more resilient than some people think.
If you keep it in the fridge then, ideally, you should refresh it every so often, e.g. every 7 days or so. However, you can neglect it for longer: just take off any yukky looking skin which may have formed, and spoon out a bit from underneath.
To refresh: stir it and add around 2-3 times flour and the same weight of water to have a 100% hydrated leaven.
So, if you have 50g starter, then add 100g of flour and 100g of water and stir. Wait around 12 hours at room temperature before using in a recipe. If it’s looking a bit stale, then repeat, with 50g of the refreshed starter, more flour+water. etc.
That’s it.