Measure out the starter, add water into the same jar. Stir to combine, add flour and combine again.
Cover loosely with a piece of baking paper or similar and keep draft free for 4 hours until it is bubbling and more than doubled. Use as directed in sourdough recipes of choice.
To make a smaller or larger amount of levain for a recipe that requires a different amount of levain, the ratio is 1 part starter to two parts water and two parts flour.
To make a levain that will take longer to develop and potentially be more active, you can use the following ratios. 1 part starter; 4 parts water, 4 parts flour. This will increase development time to around 8 hours. This can be quite convenient if you are looking to make a lot more dough, or need to be doing something else rather than worrying about the 4 hour time limit on the quantity in this initial recipe.