Day 1 - In your jar, mix together the flour and water, try to be accurate when weighing out the ingredients! I recommend using scales for this. Cover the jar with the lid but don't screw it on tight. Leave the jar alone undisturbed for the next 24- 48 hours. If it's winter, try to pick a warmer spot.
Day 2 - if you're making the starter during summer or if your house is quite warm, if may be time to feed your starter. However if it's winter, continue to leave your starter be! To feed your starter, discard half (50g) the starter - this portion that gets thrown out is called the "sourdough discard" and can be used in other recipes to reduce waste. Add another 50g flour and 50g water and mix thoroughly. Replace the lid loosely again and leave it alone for another 24 hrs. The starter will bubble up and rise as the yeast gather and grow. When it's hungry it will fall back down.
Day 3, 4, 5, 6 - feed your starter each day from day 3 onwards. Repeat the same process of discarding half and mixing in another 50g of fresh flour and water.
Day 7 - The starter will look more bubbly than before now that it's had time to establish a healthy yeast colony. The texture will look moussey, air-y and spongey. However it might not be strong enough for bread yet. A quick test if to grab a small dollop and place it in a bowl of water, if it's floating it can be used to bake bread. If not, continue to feed your starter everyday and try the float test in 1-2 week's time! In the meantime, you can use your sourdough discard in other fun recipes to reduce wastage!